Version: 260.2.4

83ad6042a7 & b267cd6fb4

Does not include test code to avoid risking merge conflicts.

(cherry picked from commit 415585a30d74fcae61f581808220a7aaeca3eaf5)
(cherry picked from commit e36628a6d436ea08d6d31441c101a88a5504c515)

Directory.Packages.props

@@ -57,7 +57,7 @@
     <PackageVersion Include="SharpZstd.Interop" Version="1.5.2-beta2" />
     <PackageVersion Include="SixLabors.ImageSharp" Version="3.1.7" />
     <PackageVersion Include="SpaceWizards.HttpListener" Version="0.1.1" />
-    <PackageVersion Include="SpaceWizards.NFluidsynth" Version="0.1.1" />
+    <PackageVersion Include="SpaceWizards.NFluidsynth" Version="0.2.2" />
     <PackageVersion Include="SpaceWizards.SharpFont" Version="1.0.2" />
     <PackageVersion Include="SpaceWizards.Sodium" Version="0.2.1" />
     <PackageVersion Include="TerraFX.Interop.Windows" Version="10.0.26100.1" />

MSBuild/Robust.Engine.Version.props

@@ -1,4 +1,4 @@
 <Project>
 
-    <!-- This file automatically reset by Tools/version.py -->
+    <!-- This file automatically reset by Tools/version.py -->
 

RELEASE-NOTES.md

@@ -54,6 +54,128 @@ END TEMPLATE-->
 *None yet*
 
 
+## 260.2.4
+
+
+## 260.2.3
+
+
+## 260.2.2
+
+
+## 260.2.1
+
+
+## 260.2.0
+
+### New features
+
+* Add `StringBuilder.Insert(int, string)` to sandbox.
+* Add the WorldNormal to the StartCollideEvent.
+
+
+## 260.1.0
+
+### New features
+
+* `ComponentFactory` is now exposed to `EntitySystem` as `Factory`
+
+### Other
+
+* Cleanup warnings in PLacementManager
+* Cleanup warnings in Clide.Sprite
+
+## 260.0.0
+
+### Breaking changes
+
+* Fix / change `StartCollideEvent.WorldPoint` to return all points for the collision which may be up to 2 instead of 1.
+
+### New features
+
+* Add SpriteSystem dependency to VisualizerSystem.
+* Add Vertical property to progress bars
+* Add some `EntProtoId` overloads for group entity spawn methods.
+
+
+## 259.0.0
+
+### Breaking changes
+
+* TileChangedEvent now has an array of tile changed entries rather than raising an individual event for every single tile changed.
+
+### Other
+
+* `Entity<T>` methods were marked as `readonly` as appropriate.
+
+
+## 258.0.1
+
+### Bugfixes
+
+* Fix static physics bodies not generating contacts if they spawn onto sleeping bodies.
+
+
+## 258.0.0
+
+### Breaking changes
+
+* `IMarkupTag` and related methods in `MarkupTagManager` have been obsoleted and should be replaced with the new `IMarkupTagHandler` interface. Various engine tags (e.g., `BoldTag`, `ColorTag`, etc) no longer implement the old interface.
+
+### New features
+
+* Add IsValidPath to ResPath and make some minor performance improvements.
+
+### Bugfixes
+
+* OutputPanel and RichTextLabel now remove controls associated with rich text tags when the text is updated.
+* Fix `SpriteComponent.Visible` datafield not being read from yaml.
+* Fix container state handling not forcing inserts.
+
+### Other
+
+* `SpriteSystem.LayerMapReserve()` no longer throws an exception if the specified layer already exists. This makes it behave like the obsoleted `SpriteComponent.LayerMapReserveBlank()`.
+
+
+## 257.0.2
+
+### Bugfixes
+
+* Fix unshaded sprite layers not rendering correctly.
+
+
+## 257.0.1
+
+### Bugfixes
+
+* Fix sprite layer bounding box calculations. This was causing various sprite rendering & render-tree lookup issues.
+
+
+## 257.0.0
+
+### Breaking changes
+
+* The client will now automatically pause any entities that leave their PVS range.
+* Contacts for terminating entities no longer raise wake events.
+
+### New features
+
+* Added `IPrototypeManager.IsIgnored()` for checking whether a given prototype kind has been marked as ignored via `RegisterIgnore()`.
+* Added `PoolManager` & `TestPair` classes to `Robust.UnitTesting`. These classes make it easier to create & use pooled server/client instance pairs in integration tests.
+* Catch NotYamlSerializable DataFields with an analyzer.
+* Optimized RSI preloading and texture atlas creation.
+
+### Bugfixes
+
+* Fix clients unintentionally un-pausing paused entities that re-enter pvs range
+
+### Other
+
+* The yaml prototype id serialiser now provides better feedback when trying to validate an id for a prototype kind that has been ignored via `IPrototypeManager.RegisterIgnore()`
+* Several SpriteComponent methods have been marked as obsolete, and should be replaced with new methods in SpriteSystem.
+* Rotation events no longer check for grid traversal.
+
+
 ## 256.0.0
 
 ### Breaking changes

Robust.Analyzers.Tests/DataDefinitionAnalyzerTest.cs

@@ -21,47 +21,53 @@ public sealed class DataDefinitionAnalyzerTest
             },
         };
 
+        test.TestState.Sources.Add(("TestTypeDefs.cs", TestTypeDefs));
+
         // ExpectedDiagnostics cannot be set, so we need to AddRange here...
         test.TestState.ExpectedDiagnostics.AddRange(expected);
 
         return test.RunAsync();
     }
 
+    private const string TestTypeDefs = """
+        using System;
+
+        namespace Robust.Shared.ViewVariables
+        {
+            public sealed class ViewVariablesAttribute : Attribute
+            {
+                public readonly VVAccess Access = VVAccess.ReadOnly;
+
+                public ViewVariablesAttribute() { }
+
+                public ViewVariablesAttribute(VVAccess access)
+                {
+                    Access = access;
+                }
+            }
+            public enum VVAccess : byte
+            {
+                ReadOnly = 0,
+                ReadWrite = 1,
+            }
+        }
+
+        namespace Robust.Shared.Serialization.Manager.Attributes
+        {
+            public class DataFieldBaseAttribute : Attribute;
+            public class DataFieldAttribute : DataFieldBaseAttribute;
+            public sealed class DataDefinitionAttribute : Attribute;
+            public sealed class NotYamlSerializableAttribute : Attribute;
+        }
+    """;
+
     [Test]
-    public async Task Test()
+    public async Task NoVVReadOnlyTest()
     {
         const string code = """
-            using System;
             using Robust.Shared.ViewVariables;
             using Robust.Shared.Serialization.Manager.Attributes;
 
-            namespace Robust.Shared.ViewVariables
-            {
-                public sealed class ViewVariablesAttribute : Attribute
-                {
-                    public readonly VVAccess Access = VVAccess.ReadOnly;
-
-                    public ViewVariablesAttribute() { }
-
-                    public ViewVariablesAttribute(VVAccess access)
-                    {
-                        Access = access;
-                    }
-                }
-                public enum VVAccess : byte
-                {
-                    ReadOnly = 0,
-                    ReadWrite = 1,
-                }
-            }
-
-            namespace Robust.Shared.Serialization.Manager.Attributes
-            {
-                public class DataFieldBaseAttribute : Attribute;
-                public class DataFieldAttribute : DataFieldBaseAttribute;
-                public sealed class DataDefinitionAttribute : Attribute;
-            }
-
             [DataDefinition]
             public sealed partial class Foo
             {
@@ -83,8 +89,8 @@ public sealed class DataDefinitionAnalyzerTest
             """;
 
         await Verifier(code,
-            // /0/Test0.cs(35,17): info RA0028: Data field Bad in data definition Foo has ViewVariables attribute with ReadWrite access, which is redundant
-            VerifyCS.Diagnostic(DataDefinitionAnalyzer.DataFieldNoVVReadWriteRule).WithSpan(35, 17, 35, 50).WithArguments("Bad", "Foo")
+            // /0/Test0.cs(7,17): info RA0028: Data field Bad in data definition Foo has ViewVariables attribute with ReadWrite access, which is redundant
+            VerifyCS.Diagnostic(DataDefinitionAnalyzer.DataFieldNoVVReadWriteRule).WithSpan(7, 17, 7, 50).WithArguments("Bad", "Foo")
         );
     }
 
@@ -92,16 +98,8 @@ public sealed class DataDefinitionAnalyzerTest
     public async Task ReadOnlyFieldTest()
     {
         const string code = """
-            using System;
             using Robust.Shared.Serialization.Manager.Attributes;
 
-            namespace Robust.Shared.Serialization.Manager.Attributes
-            {
-                public class DataFieldBaseAttribute : Attribute;
-                public class DataFieldAttribute : DataFieldBaseAttribute;
-                public sealed class DataDefinitionAttribute : Attribute;
-            }
-
             [DataDefinition]
             public sealed partial class Foo
             {
@@ -114,8 +112,8 @@ public sealed class DataDefinitionAnalyzerTest
             """;
 
         await Verifier(code,
-            // /0/Test0.cs(15,12): error RA0019: Data field Bad in data definition Foo is readonly
-            VerifyCS.Diagnostic(DataDefinitionAnalyzer.DataFieldWritableRule).WithSpan(15, 12, 15, 20).WithArguments("Bad", "Foo")
+            // /0/Test0.cs(7,12): error RA0019: Data field Bad in data definition Foo is readonly
+            VerifyCS.Diagnostic(DataDefinitionAnalyzer.DataFieldWritableRule).WithSpan(7, 12, 7, 20).WithArguments("Bad", "Foo")
         );
     }
 
@@ -123,16 +121,8 @@ public sealed class DataDefinitionAnalyzerTest
     public async Task ReadOnlyPropertyTest()
     {
         const string code = """
-            using System;
             using Robust.Shared.Serialization.Manager.Attributes;
 
-            namespace Robust.Shared.Serialization.Manager.Attributes
-            {
-                public class DataFieldBaseAttribute : Attribute;
-                public class DataFieldAttribute : DataFieldBaseAttribute;
-                public sealed class DataDefinitionAttribute : Attribute;
-            }
-
             [DataDefinition]
             public sealed partial class Foo
             {
@@ -145,8 +135,40 @@ public sealed class DataDefinitionAnalyzerTest
             """;
 
         await Verifier(code,
-            // /0/Test0.cs(15,20): error RA0020: Data field property Bad in data definition Foo does not have a setter
-            VerifyCS.Diagnostic(DataDefinitionAnalyzer.DataFieldPropertyWritableRule).WithSpan(15, 20, 15, 28).WithArguments("Bad", "Foo")
+            // /0/Test0.cs(7,20): error RA0020: Data field property Bad in data definition Foo does not have a setter
+            VerifyCS.Diagnostic(DataDefinitionAnalyzer.DataFieldPropertyWritableRule).WithSpan(7, 20, 7, 28).WithArguments("Bad", "Foo")
+        );
+    }
+
+    [Test]
+    public async Task NotYamlSerializableTest()
+    {
+        const string code = """
+            using Robust.Shared.Serialization.Manager.Attributes;
+
+            [NotYamlSerializable]
+            public sealed class NotSerializableClass { }
+
+            [DataDefinition]
+            public sealed partial class Foo
+            {
+                [DataField]
+                public NotSerializableClass BadField;
+
+                [DataField]
+                public NotSerializableClass BadProperty { get; set; }
+
+                public NotSerializableClass GoodField; // Not a DataField, not a problem
+
+                public NotSerializableClass GoodProperty { get; set; } // Not a DataField, not a problem
+            }
+            """;
+
+        await Verifier(code,
+            // /0/Test0.cs(10,12): error RA0033: Data field BadField in data definition Foo is type NotSerializableClass, which is not YAML serializable
+            VerifyCS.Diagnostic(DataDefinitionAnalyzer.DataFieldYamlSerializableRule).WithSpan(10, 12, 10, 32).WithArguments("BadField", "Foo", "NotSerializableClass"),
+            // /0/Test0.cs(13,12): error RA0033: Data field BadProperty in data definition Foo is type NotSerializableClass, which is not YAML serializable
+            VerifyCS.Diagnostic(DataDefinitionAnalyzer.DataFieldYamlSerializableRule).WithSpan(13, 12, 13, 32).WithArguments("BadProperty", "Foo", "NotSerializableClass")
         );
     }
 }

Robust.Analyzers/DataDefinitionAnalyzer.cs

@@ -18,6 +18,7 @@ public sealed class DataDefinitionAnalyzer : DiagnosticAnalyzer
     private const string ImplicitDataDefinitionNamespace = "Robust.Shared.Serialization.Manager.Attributes.ImplicitDataDefinitionForInheritorsAttribute";
     private const string DataFieldBaseNamespace = "Robust.Shared.Serialization.Manager.Attributes.DataFieldBaseAttribute";
     private const string ViewVariablesNamespace = "Robust.Shared.ViewVariables.ViewVariablesAttribute";
+    private const string NotYamlSerializableName = "Robust.Shared.Serialization.Manager.Attributes.NotYamlSerializableAttribute";
     private const string DataFieldAttributeName = "DataField";
     private const string ViewVariablesAttributeName = "ViewVariables";
 
@@ -81,9 +82,19 @@ public sealed class DataDefinitionAnalyzer : DiagnosticAnalyzer
         "Make sure to remove the ViewVariables attribute."
     );
 
+    public static readonly DiagnosticDescriptor DataFieldYamlSerializableRule = new(
+        Diagnostics.IdDataFieldYamlSerializable,
+        "Data field type is not YAML serializable",
+        "Data field {0} in data definition {1} is type {2}, which is not YAML serializable",
+        "Usage",
+        DiagnosticSeverity.Error,
+        true,
+        "Make sure to use a type that is YAML serializable."
+    );
+
     public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(
         DataDefinitionPartialRule, NestedDataDefinitionPartialRule, DataFieldWritableRule, DataFieldPropertyWritableRule,
-        DataFieldRedundantTagRule, DataFieldNoVVReadWriteRule
+        DataFieldRedundantTagRule, DataFieldNoVVReadWriteRule, DataFieldYamlSerializableRule
     );
 
     public override void Initialize(AnalysisContext context)
@@ -164,6 +175,19 @@ public sealed class DataDefinitionAnalyzer : DiagnosticAnalyzer
                 TryGetAttributeLocation(field, ViewVariablesAttributeName, out var location);
                 context.ReportDiagnostic(Diagnostic.Create(DataFieldNoVVReadWriteRule, location, fieldSymbol.Name, type.Name));
             }
+
+            if (context.SemanticModel.GetSymbolInfo(field.Declaration.Type).Symbol is not ITypeSymbol fieldTypeSymbol)
+                continue;
+
+            if (IsNotYamlSerializable(fieldSymbol, fieldTypeSymbol))
+            {
+                context.ReportDiagnostic(Diagnostic.Create(DataFieldYamlSerializableRule,
+                    (context.Node as FieldDeclarationSyntax)?.Declaration.Type.GetLocation(),
+                    fieldSymbol.Name,
+                    type.Name,
+                    fieldTypeSymbol.MetadataName
+                ));
+            }
         }
     }
 
@@ -201,6 +225,19 @@ public sealed class DataDefinitionAnalyzer : DiagnosticAnalyzer
             TryGetAttributeLocation(property, ViewVariablesAttributeName, out var location);
             context.ReportDiagnostic(Diagnostic.Create(DataFieldNoVVReadWriteRule, location, propertySymbol.Name, type.Name));
         }
+
+        if (context.SemanticModel.GetSymbolInfo(property.Type).Symbol is not ITypeSymbol propertyTypeSymbol)
+            return;
+
+        if (IsNotYamlSerializable(propertySymbol, propertyTypeSymbol))
+        {
+            context.ReportDiagnostic(Diagnostic.Create(DataFieldYamlSerializableRule,
+                (context.Node as PropertyDeclarationSyntax)?.Type.GetLocation(),
+                propertySymbol.Name,
+                type.Name,
+                propertyTypeSymbol.Name
+            ));
+        }
     }
 
     private static bool IsReadOnlyDataField(ITypeSymbol type, ISymbol field)
@@ -383,6 +420,14 @@ public sealed class DataDefinitionAnalyzer : DiagnosticAnalyzer
         return (VVAccess)accessByte == VVAccess.ReadWrite;
     }
 
+    private static bool IsNotYamlSerializable(ISymbol field, ITypeSymbol type)
+    {
+        if (!IsDataField(field, out _, out _))
+            return false;
+
+        return HasAttribute(type, NotYamlSerializableName);
+    }
+
     private static bool IsImplicitDataDefinition(ITypeSymbol type)
     {
         if (HasAttribute(type, ImplicitDataDefinitionNamespace))

Robust.Client.WebView/Cef/Program.cs

@@ -6,7 +6,7 @@ using Xilium.CefGlue;
 
 namespace Robust.Client.WebView.Cef
 {
-    public static class Program
+    internal static class Program
     {
         // This was supposed to be the main entry for the subprocess program... It doesn't work.
         public static int Main(string[] args)

Robust.Client.WebView/Cef/WebViewManagerCef.cs

@@ -5,6 +5,7 @@ using System.Net;
 using System.Reflection;
 using System.Text;
 using Robust.Client.Console;
+using Robust.Client.Utility;
 using Robust.Shared.Configuration;
 using Robust.Shared.ContentPack;
 using Robust.Shared.IoC;
@@ -24,6 +25,7 @@ namespace Robust.Client.WebView.Cef
 
         [Dependency] private readonly IDependencyCollection _dependencyCollection = default!;
         [Dependency] private readonly IPrototypeManager _prototypeManager = default!;
+        [Dependency] private readonly IGameControllerInternal _gameController = default!;
         [Dependency] private readonly IResourceManagerInternal _resourceManager = default!;
         [Dependency] private readonly IClientConsoleHost _consoleHost = default!;
         [Dependency] private readonly IConfigurationManager _cfg = default!;
@@ -61,7 +63,10 @@ namespace Robust.Client.WebView.Cef
 
             var cachePath = "";
             if (_resourceManager.UserData is WritableDirProvider userData)
-                cachePath = userData.GetFullPath(new ResPath("/cef_cache"));
+            {
+                var rootDir = UserDataDir.GetRootUserDataDir(_gameController);
+                cachePath = Path.Combine(rootDir, "cef_cache", "0");
+            }
 
             var settings = new CefSettings()
             {

Robust.Client/ComponentTrees/SpriteTreeSystem.cs

@@ -2,6 +2,7 @@ using System.Numerics;
 using Robust.Client.GameObjects;
 using Robust.Shared.ComponentTrees;
 using Robust.Shared.GameObjects;
+using Robust.Shared.IoC;
 using Robust.Shared.Maths;
 using Robust.Shared.Physics;
 
@@ -9,25 +10,7 @@ namespace Robust.Client.ComponentTrees;
 
 public sealed class SpriteTreeSystem : ComponentTreeSystem<SpriteTreeComponent, SpriteComponent>
 {
-    public override void Initialize()
-    {
-        base.Initialize();
-        SubscribeLocalEvent<SpriteComponent, QueueSpriteTreeUpdateEvent>(OnQueueUpdate);
-    }
-
-    private void OnQueueUpdate(EntityUid uid, SpriteComponent component, ref QueueSpriteTreeUpdateEvent args)
-        => QueueTreeUpdate(uid, component, args.Xform);
-
-    // TODO remove this when finally ECSing sprite components
-    [ByRefEvent]
-    internal readonly struct QueueSpriteTreeUpdateEvent
-    {
-        public readonly TransformComponent Xform;
-        public QueueSpriteTreeUpdateEvent(TransformComponent xform)
-        {
-            Xform = xform;
-        }
-    }
+    [Dependency] private readonly SpriteSystem _sprite = default!;
 
     #region Component Tree Overrides
     protected override bool DoFrameUpdate => true;
@@ -36,6 +19,11 @@ public sealed class SpriteTreeSystem : ComponentTreeSystem<SpriteTreeComponent,
     protected override int InitialCapacity => 1024;
 
     protected override Box2 ExtractAabb(in ComponentTreeEntry<SpriteComponent> entry, Vector2 pos, Angle rot)
-        => entry.Component.CalculateRotatedBoundingBox(pos, rot, default).CalcBoundingBox();
+    {
+        // TODO SPRITE optimize this
+        // Because the just take the BB of the rotated BB, I'mt pretty sure we do a lot of unnecessary maths.
+        return _sprite.CalculateBounds((entry.Uid, entry.Component), pos, rot, default).CalcBoundingBox();
+    }
+
     #endregion
 }

Robust.Client/GameController/GameController.cs

@@ -387,7 +387,7 @@ namespace Robust.Client
 
             _prof.Initialize();
 
-            _resManager.Initialize(Options.LoadConfigAndUserData ? userDataDir : null);
+            _resManager.Initialize(Options.LoadConfigAndUserData ? userDataDir : null, hideUserDataDir: true);
 
             var mountOptions = _commandLineArgs != null
                 ? MountOptions.Merge(_commandLineArgs.MountOptions, Options.MountOptions)

Robust.Client/GameObjects/Components/Renderable/IRenderableComponent.cs

@@ -1,9 +1,11 @@
-using Robust.Shared.GameObjects;
+using System;
+using Robust.Shared.GameObjects;
 using Robust.Shared.Map;
 using Robust.Shared.Maths;
 
 namespace Robust.Client.GameObjects
 {
+    [Obsolete]
     public partial interface IRenderableComponent : IComponent
     {
         int DrawDepth { get; set; }

Robust.Client/GameObjects/Components/Renderable/ISpriteLayer.cs

@@ -1,6 +1,5 @@
 using System.Numerics;
 using Robust.Client.Graphics;
-using Robust.Shared.Graphics;
 using Robust.Shared.Graphics.RSI;
 using Robust.Shared.Maths;
 

Robust.Client/GameObjects/EntitySystems/ContainerSystem.cs

@@ -1,15 +1,14 @@
-using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
 using Robust.Shared.Collections;
 using Robust.Shared.Containers;
 using Robust.Shared.GameObjects;
 using Robust.Shared.GameStates;
 using Robust.Shared.IoC;
 using Robust.Shared.Map;
-using Robust.Shared.Utility;
-using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
-using System.Linq;
 using Robust.Shared.Serialization;
+using Robust.Shared.Utility;
 using static Robust.Shared.Containers.ContainerManagerComponent;
 
 namespace Robust.Client.GameObjects
@@ -58,7 +57,7 @@ namespace Robust.Client.GameObjects
             if (!RemoveExpectedEntity(meta.NetEntity, out var container))
                 return;
 
-            Insert((uid, TransformQuery.GetComponent(uid), MetaQuery.GetComponent(uid), null), container);
+            Insert((uid, TransformQuery.GetComponent(uid), MetaQuery.GetComponent(uid), null), container, force: true);
         }
 
         public override void ShutdownContainer(BaseContainer container)
@@ -232,7 +231,7 @@ namespace Robust.Client.GameObjects
                 return;
             }
 
-            Insert(message.Entity, container);
+            Insert(message.Entity, container, force: true);
         }
 
         public void AddExpectedEntity(NetEntity netEntity, BaseContainer container)

Robust.Client/GameObjects/EntitySystems/SpriteSystem.Bounds.cs

@@ -0,0 +1,149 @@
+using System;
+using System.Linq;
+using System.Numerics;
+using Robust.Client.Graphics;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Graphics.RSI;
+using Robust.Shared.Maths;
+using Robust.Shared.Utility;
+using static Robust.Client.GameObjects.SpriteComponent;
+
+namespace Robust.Client.GameObjects;
+
+// This partial class contains code related to updating a sprites bounding boxes and its position in the sprite tree.
+public sealed partial class SpriteSystem
+{
+    /// <summary>
+    /// Get a sprite's local bounding box. The returned bounds do factor in the sprite's scale but not the rotation or
+    /// offset.
+    /// </summary>
+    public Box2 GetLocalBounds(Entity<SpriteComponent> sprite)
+    {
+        if (!sprite.Comp.BoundsDirty)
+        {
+            DebugTools.Assert(sprite.Comp.Layers.All(x => !x.BoundsDirty || !x.Drawn));
+            return sprite.Comp._bounds;
+        }
+
+        var bounds = new Box2();
+        foreach (var layer in sprite.Comp.Layers)
+        {
+            if (layer.Drawn)
+                bounds = bounds.Union(GetLocalBounds(layer));
+        }
+
+        sprite.Comp._bounds = bounds;
+        sprite.Comp.BoundsDirty = false;
+        return sprite.Comp._bounds;
+    }
+
+    /// <summary>
+    /// Get a layer's local bounding box relative to its owning sprite. Unlike the sprite variant of this method, this
+    /// does account for the layer's rotation and offset.
+    /// </summary>
+    public Box2 GetLocalBounds(Layer layer)
+    {
+        if (!layer.BoundsDirty)
+        {
+            DebugTools.Assert(layer.Bounds.EqualsApprox(CalculateLocalBounds(layer)));
+            return layer.Bounds;
+        }
+
+        layer.Bounds = CalculateLocalBounds(layer);
+        layer.BoundsDirty = false;
+        return layer.Bounds;
+    }
+
+    internal Box2 CalculateLocalBounds(Layer layer)
+    {
+        var textureSize = (Vector2) layer.PixelSize / EyeManager.PixelsPerMeter;
+        var longestSide = MathF.Max(textureSize.X, textureSize.Y);
+        var longestRotatedSide = Math.Max(longestSide, (textureSize.X + textureSize.Y) / MathF.Sqrt(2));
+
+        Vector2 size;
+        var sprite = layer.Owner.Comp;
+
+        // If this layer has any form of arbitrary rotation, return a bounding box big enough to cover
+        // any possible rotation.
+        if (layer._rotation != 0)
+        {
+            size = new Vector2(longestRotatedSide, longestRotatedSide);
+            return Box2.CenteredAround(layer.Offset, size * layer._scale);
+        }
+
+        var snapToCardinals = sprite.SnapCardinals;
+        if (sprite.GranularLayersRendering && layer.RenderingStrategy != LayerRenderingStrategy.UseSpriteStrategy)
+        {
+            snapToCardinals = layer.RenderingStrategy == LayerRenderingStrategy.SnapToCardinals;
+        }
+
+        if (snapToCardinals)
+        {
+            // Snapping to cardinals only makes sense for 1-directional layers/sprites
+            DebugTools.Assert(layer._actualState == null || layer._actualState.RsiDirections == RsiDirectionType.Dir1);
+
+            // We won't know the actual direction it snaps to, so we ahve to assume the box is given by the longest side.
+            size = new Vector2(longestSide, longestSide);
+            return Box2.CenteredAround(layer.Offset, size * layer._scale);
+        }
+
+        // Build the bounding box based on how many directions the sprite has
+        size = (layer._actualState?.RsiDirections) switch
+        {
+            RsiDirectionType.Dir4 => new Vector2(longestSide, longestSide),
+            RsiDirectionType.Dir8 => new Vector2(longestRotatedSide, longestRotatedSide),
+            _ => textureSize
+        };
+
+        return Box2.CenteredAround(layer.Offset, size * layer._scale);
+    }
+
+    /// <summary>
+    /// Gets a sprite's bounding box in world coordinates.
+    /// </summary>
+    public Box2Rotated CalculateBounds(Entity<SpriteComponent> sprite, Vector2 worldPos, Angle worldRot, Angle eyeRot)
+    {
+        // fast check for invisible sprites
+        if (!sprite.Comp.Visible || sprite.Comp.Layers.Count == 0)
+            return new Box2Rotated(new Box2(worldPos, worldPos), Angle.Zero, worldPos);
+
+        // We need to modify world rotation so that it lies between 0 and 2pi.
+        // This matters for 4 or 8 directional sprites deciding which quadrant (octant?) they lie in.
+        // the 0->2pi convention is set by the sprite-rendering code that selects the layers.
+        // See RenderInternal().
+
+        worldRot = worldRot.Reduced();
+        if (worldRot.Theta < 0)
+            worldRot = new Angle(worldRot.Theta + Math.Tau);
+
+        // Next, what we do is take the box2 and apply the sprite's transform, and then the entity's transform. We
+        // could do this via Matrix3.TransformBox, but that only yields bounding boxes. So instead we manually
+        // transform our box by the combination of these matrices:
+
+        var finalRotation = sprite.Comp.NoRotation
+            ? sprite.Comp.Rotation - eyeRot
+            : sprite.Comp.Rotation + worldRot;
+
+        var bounds = GetLocalBounds(sprite);
+
+        // slightly faster path if offset == 0 (true for 99.9% of sprites)
+        if (sprite.Comp.Offset == Vector2.Zero)
+            return new Box2Rotated(bounds.Translated(worldPos), finalRotation, worldPos);
+
+        var adjustedOffset = sprite.Comp.NoRotation
+            ? (-eyeRot).RotateVec(sprite.Comp.Offset)
+            : worldRot.RotateVec(sprite.Comp.Offset);
+
+        var position = adjustedOffset + worldPos;
+        return new Box2Rotated(bounds.Translated(position), finalRotation, position);
+    }
+
+    private void DirtyBounds(Entity<SpriteComponent> sprite)
+    {
+        sprite.Comp.BoundsDirty = true;
+        foreach (var layer in sprite.Comp.Layers)
+        {
+            layer.BoundsDirty = true;
+        }
+    }
+}

Robust.Client/GameObjects/EntitySystems/SpriteSystem.Component.cs

@@ -1,4 +1,8 @@
-using System.Linq;
+using System;
+using System.Collections.Generic;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Maths;
+using Robust.Shared.Utility;
 
 namespace Robust.Client.GameObjects;
 
@@ -41,4 +45,66 @@ public sealed partial class SpriteSystem
         layer.AnimationTimeLeft = (float) -(time % state.TotalDelay);
         layer.AnimationFrame = 0;
     }
+
+    public void CopySprite(Entity<SpriteComponent?> source, Entity<SpriteComponent?> target)
+    {
+        if (!Resolve(source.Owner, ref source.Comp))
+            return;
+
+        if (!Resolve(target.Owner, ref target.Comp))
+            return;
+
+        target.Comp._baseRsi = source.Comp._baseRsi;
+        target.Comp._bounds = source.Comp._bounds;
+        target.Comp._visible = source.Comp._visible;
+        target.Comp.color = source.Comp.color;
+        target.Comp.offset = source.Comp.offset;
+        target.Comp.rotation = source.Comp.rotation;
+        target.Comp.scale = source.Comp.scale;
+        target.Comp.LocalMatrix = Matrix3Helpers.CreateTransform(
+            in target.Comp.offset,
+            in target.Comp.rotation,
+            in target
+            .Comp.scale);
+
+        target.Comp.drawDepth = source.Comp.drawDepth;
+        target.Comp.NoRotation = source.Comp.NoRotation;
+        target.Comp.DirectionOverride = source.Comp.DirectionOverride;
+        target.Comp.EnableDirectionOverride = source.Comp.EnableDirectionOverride;
+        target.Comp.Layers = new List<SpriteComponent.Layer>(source.Comp.Layers.Count);
+        foreach (var otherLayer in source.Comp.Layers)
+        {
+            var layer = new SpriteComponent.Layer(otherLayer, target.Comp);
+            layer.Index = target.Comp.Layers.Count;
+            layer.Owner = target!;
+            target.Comp.Layers.Add(layer);
+        }
+
+        target.Comp.IsInert = source.Comp.IsInert;
+        target.Comp.LayerMap = source.Comp.LayerMap.ShallowClone();
+        target.Comp.PostShader = source.Comp.PostShader is {Mutable: true}
+            ? source.Comp.PostShader.Duplicate()
+            : source.Comp.PostShader;
+
+        target.Comp.RenderOrder = source.Comp.RenderOrder;
+        target.Comp.GranularLayersRendering = source.Comp.GranularLayersRendering;
+
+        DirtyBounds(target!);
+        _tree.QueueTreeUpdate(target!);
+    }
+
+    /// <summary>
+    /// Adds a sprite to a queue that will update <see cref="SpriteComponent.IsInert"/> next frame.
+    /// </summary>
+    public void QueueUpdateIsInert(Entity<SpriteComponent> sprite)
+    {
+        if (sprite.Comp._inertUpdateQueued)
+            return;
+
+        sprite.Comp._inertUpdateQueued = true;
+        _inertUpdateQueue.Enqueue(sprite);
+    }
+
+    [Obsolete("Use QueueUpdateIsInert")]
+    public void QueueUpdateInert(EntityUid uid, SpriteComponent sprite) => QueueUpdateIsInert(new (uid, sprite));
 }

Robust.Client/GameObjects/EntitySystems/SpriteSystem.Helpers.cs

@@ -1,11 +1,153 @@
+using System;
+using System.Collections.Generic;
 using System.Numerics;
+using JetBrains.Annotations;
+using Robust.Client.Graphics;
+using Robust.Client.ResourceManagement;
 using Robust.Shared.GameObjects;
 using Robust.Shared.Map;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Utility;
 
 namespace Robust.Client.GameObjects;
 
+// This partial class contains various public helper methods, including methods for extracting textures/icons from
+// sprite specifiers and entity prototypes.
 public sealed partial class SpriteSystem
 {
+    private readonly Dictionary<string, IRsiStateLike> _cachedPrototypeIcons = new();
+
+    public Texture Frame0(EntityPrototype prototype)
+    {
+        return GetPrototypeIcon(prototype).Default;
+    }
+
+    public Texture Frame0(SpriteSpecifier specifier)
+    {
+        return RsiStateLike(specifier).Default;
+    }
+
+    public IRsiStateLike RsiStateLike(SpriteSpecifier specifier)
+    {
+        switch (specifier)
+        {
+            case SpriteSpecifier.Texture tex:
+                return GetTexture(tex);
+
+            case SpriteSpecifier.Rsi rsi:
+                return GetState(rsi);
+
+            case SpriteSpecifier.EntityPrototype prototypeIcon:
+                return GetPrototypeIcon(prototypeIcon.EntityPrototypeId);
+
+            default:
+                throw new NotSupportedException();
+        }
+    }
+
+    public Texture GetIcon(IconComponent icon)
+    {
+        return GetState(icon.Icon).Frame0;
+    }
+
+    /// <summary>
+    ///     Returns an icon for a given <see cref="EntityPrototype"/> ID, or a fallback in case of an error.
+    ///     This method caches the result based on the prototype identifier.
+    /// </summary>
+    public IRsiStateLike GetPrototypeIcon(string prototype)
+    {
+        // Check if this prototype has been cached before, and if so return the result.
+        if (_cachedPrototypeIcons.TryGetValue(prototype, out var cachedResult))
+            return cachedResult;
+
+        if (!_proto.TryIndex<EntityPrototype>(prototype, out var entityPrototype))
+        {
+            // The specified prototype doesn't exist, return the fallback "error" sprite.
+            _sawmill.Error("Failed to load PrototypeIcon {0}", prototype);
+            return GetFallbackState();
+        }
+
+        // Generate the icon and cache it in case it's ever needed again.
+        var result = GetPrototypeIcon(entityPrototype);
+        _cachedPrototypeIcons[prototype] = result;
+
+        return result;
+    }
+
+    /// <summary>
+    ///     Returns an icon for a given <see cref="EntityPrototype"/> ID, or a fallback in case of an error.
+    ///     This method does NOT cache the result.
+    /// </summary>
+    public IRsiStateLike GetPrototypeIcon(EntityPrototype prototype)
+    {
+        // IconComponent takes precedence. If it has a valid icon, return that. Otherwise, continue as normal.
+        if (prototype.Components.TryGetValue("Icon", out var compData)
+            && compData.Component is IconComponent icon)
+        {
+            return GetIcon(icon);
+        }
+
+        // If the prototype doesn't have a SpriteComponent, then there's nothing we can do but return the fallback.
+        if (!prototype.Components.ContainsKey("Sprite"))
+        {
+            return GetFallbackState();
+        }
+
+        // Finally, we use spawn a dummy entity to get its icon.
+        var dummy = Spawn(prototype.ID, MapCoordinates.Nullspace);
+        var spriteComponent = EnsureComp<SpriteComponent>(dummy);
+        var result = spriteComponent.Icon ?? GetFallbackState();
+        Del(dummy);
+
+        return result;
+    }
+
+    [Pure]
+    public RSI.State GetFallbackState()
+    {
+        return _resourceCache.GetFallback<RSIResource>().RSI["error"];
+    }
+
+    public Texture GetFallbackTexture()
+    {
+        return _resourceCache.GetFallback<TextureResource>().Texture;
+    }
+
+    [Pure]
+    public RSI.State GetState(SpriteSpecifier.Rsi rsiSpecifier)
+    {
+        if (_resourceCache.TryGetResource<RSIResource>(
+                TextureRoot / rsiSpecifier.RsiPath,
+                out var theRsi) &&
+            theRsi.RSI.TryGetState(rsiSpecifier.RsiState, out var state))
+        {
+            return state;
+        }
+
+        _sawmill.Error("Failed to load RSI {0}", rsiSpecifier.RsiPath);
+        return GetFallbackState();
+    }
+
+    public Texture GetTexture(SpriteSpecifier.Texture texSpecifier)
+    {
+        return _resourceCache
+            .GetResource<TextureResource>(TextureRoot / texSpecifier.TexturePath)
+            .Texture;
+    }
+
+    private void OnPrototypesReloaded(PrototypesReloadedEventArgs args)
+    {
+        if (!args.TryGetModified<EntityPrototype>(out var modified))
+            return;
+
+        // Remove all changed prototypes from the cache, if they're there.
+        foreach (var prototype in modified)
+        {
+            // Let's be lazy and not regenerate them until something needs them again.
+            _cachedPrototypeIcons.Remove(prototype);
+        }
+    }
+
     /// <summary>
     /// Gets an entity's sprite position in world terms.
     /// </summary>

Robust.Client/GameObjects/EntitySystems/SpriteSystem.Layer.cs

@@ -0,0 +1,257 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Robust.Client.Graphics;
+using Robust.Client.ResourceManagement;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Utility;
+using static Robust.Client.GameObjects.SpriteComponent;
+
+namespace Robust.Client.GameObjects;
+
+// This partial class contains various public methods for managing a sprite's layers.
+// This setter methods for modifying a layer's properties are in a separate file.
+public sealed partial class SpriteSystem
+{
+    public bool LayerExists(Entity<SpriteComponent?> sprite, int index)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return false;
+
+        return index > 0 && index < sprite.Comp.Layers.Count;
+    }
+
+    public bool TryGetLayer(
+        Entity<SpriteComponent?> sprite,
+        int index,
+        [NotNullWhen(true)] out Layer? layer,
+        bool logMissing)
+    {
+        layer = null;
+
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+            return false;
+
+        if (index >= 0 && index < sprite.Comp.Layers.Count)
+        {
+            layer = sprite.Comp.Layers[index];
+            DebugTools.AssertEqual(layer.Owner, sprite!);
+            DebugTools.AssertEqual(layer.Index, index);
+            return true;
+        }
+
+        if (logMissing)
+            Log.Error($"Layer index '{index}' on entity {ToPrettyString(sprite)} does not exist! Trace:\n{Environment.StackTrace}");
+
+        return false;
+    }
+
+    public bool RemoveLayer(Entity<SpriteComponent?> sprite, int index, bool logMissing = true)
+    {
+        return RemoveLayer(sprite.Owner, index, out _, logMissing);
+    }
+
+    public bool RemoveLayer(
+        Entity<SpriteComponent?> sprite,
+        int index,
+        [NotNullWhen(true)] out Layer? layer,
+        bool logMissing = true)
+    {
+        layer = null;
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+            return false;
+
+        if (!TryGetLayer(sprite, index, out layer, logMissing))
+            return false;
+
+        sprite.Comp.Layers.RemoveAt(index);
+
+        foreach (var otherLayer in sprite.Comp.Layers[index..])
+        {
+            otherLayer.Index--;
+        }
+
+        // TODO SPRITE track inverse-mapping?
+        foreach (var (key, value) in sprite.Comp.LayerMap)
+        {
+            if (value == index)
+                sprite.Comp.LayerMap.Remove(key);
+            else if (value > index)
+            {
+                sprite.Comp.LayerMap[key]--;
+            }
+        }
+
+        layer.Owner = default;
+        layer.Index = -1;
+
+#if DEBUG
+        foreach (var otherLayer in sprite.Comp.Layers)
+        {
+            DebugTools.AssertEqual(otherLayer, sprite.Comp.Layers[otherLayer.Index]);
+        }
+#endif
+
+        sprite.Comp.BoundsDirty = true;
+        _tree.QueueTreeUpdate(sprite!);
+        QueueUpdateIsInert(sprite!);
+        return true;
+    }
+
+    #region AddLayer
+
+    /// <summary>
+    /// Add the given sprite layer. If an index is specified, this will insert the layer with the given index, resulting
+    /// in all other layers being reshuffled.
+    /// </summary>
+    public int AddLayer(Entity<SpriteComponent?> sprite, Layer layer, int? index = null)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+        {
+            layer.Index = -1;
+            layer.Owner = default;
+            return -1;
+        }
+
+        layer.Owner = sprite!;
+
+        if (index is { } i && i != sprite.Comp.Layers.Count)
+        {
+            foreach (var otherLayer in sprite.Comp.Layers[i..])
+            {
+                otherLayer.Index++;
+            }
+
+            // TODO SPRITE track inverse-mapping?
+            sprite.Comp.Layers.Insert(i, layer);
+            layer.Index = i;
+
+            foreach (var (key, value) in sprite.Comp.LayerMap)
+            {
+                if (value >= i)
+                    sprite.Comp.LayerMap[key]++;
+            }
+        }
+        else
+        {
+            layer.Index = sprite.Comp.Layers.Count;
+            sprite.Comp.Layers.Add(layer);
+        }
+
+#if DEBUG
+        foreach (var otherLayer in sprite.Comp.Layers)
+        {
+            DebugTools.AssertEqual(otherLayer, sprite.Comp.Layers[otherLayer.Index]);
+        }
+#endif
+
+        layer.BoundsDirty = true;
+        if (!layer.Blank)
+        {
+            sprite.Comp.BoundsDirty = true;
+            _tree.QueueTreeUpdate(sprite!);
+            QueueUpdateIsInert(sprite!);
+        }
+        return layer.Index;
+    }
+
+    /// <summary>
+    /// Add a layer corresponding to the given RSI state.
+    /// </summary>
+    /// <param name="sprite">The sprite</param>
+    /// <param name="stateId">The RSI state</param>
+    /// <param name="rsi">The RSI to use. If not specified, it will default to using <see cref="SpriteComponent.BaseRSI"/></param>
+    /// <param name="index">The layer index to use for the new sprite.</param>
+    /// <returns></returns>
+    public int AddRsiLayer(Entity<SpriteComponent?> sprite, RSI.StateId stateId, RSI? rsi = null, int? index = null)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return -1;
+
+        var layer = AddBlankLayer(sprite!, index);
+
+        if (rsi != null)
+            LayerSetRsi(layer, rsi, stateId);
+        else
+            LayerSetRsiState(layer, stateId);
+
+        return layer.Index;
+    }
+
+    /// <summary>
+    /// Add a layer corresponding to the given RSI state.
+    /// </summary>
+    /// <param name="sprite">The sprite</param>
+    /// <param name="state">The RSI state</param>
+    /// <param name="path">The path to the RSI.</param>
+    /// <param name="index">The layer index to use for the new sprite.</param>
+    /// <returns></returns>
+    public int AddRsiLayer(Entity<SpriteComponent?> sprite, RSI.StateId state, ResPath path, int? index = null)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return -1;
+
+        if (!_resourceCache.TryGetResource<RSIResource>(TextureRoot / path, out var res))
+            Log.Error($"Unable to load RSI '{path}'. Trace:\n{Environment.StackTrace}");
+
+        if (path.Extension != "rsi")
+            Log.Error($"Expected rsi path but got '{path}'?");
+
+        return AddRsiLayer(sprite, state, res?.RSI, index);
+    }
+
+    public int AddTextureLayer(Entity<SpriteComponent?> sprite, ResPath path, int? index = null)
+    {
+        if (_resourceCache.TryGetResource<TextureResource>(TextureRoot / path, out var texture))
+            return AddTextureLayer(sprite, texture?.Texture, index);
+
+        if (path.Extension == "rsi")
+            Log.Error($"Expected texture but got rsi '{path}', did you mean 'sprite:' instead of 'texture:'?");
+
+        Log.Error($"Unable to load texture '{path}'. Trace:\n{Environment.StackTrace}");
+        return AddTextureLayer(sprite, texture?.Texture, index);
+    }
+
+    public int AddTextureLayer(Entity<SpriteComponent?> sprite, Texture? texture, int? index = null)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return -1;
+
+        var layer = new Layer {Texture = texture};
+        return AddLayer(sprite, layer, index);
+    }
+
+    public int AddLayer(Entity<SpriteComponent?> sprite, SpriteSpecifier specifier, int? newIndex = null)
+    {
+        return specifier switch
+        {
+            SpriteSpecifier.Texture tex => AddTextureLayer(sprite, tex.TexturePath, newIndex),
+            SpriteSpecifier.Rsi rsi => AddRsiLayer(sprite, rsi.RsiState, rsi.RsiPath, newIndex),
+            _ => throw new NotImplementedException()
+        };
+    }
+
+    /// <summary>
+    /// Add a new sprite layer and populate it using the provided layer data.
+    /// </summary>
+    public int AddLayer(Entity<SpriteComponent?> sprite, PrototypeLayerData layerDatum, int? index)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return -1;
+
+        var layer = AddBlankLayer(sprite!, index);
+        LayerSetData(layer, layerDatum);
+        return layer.Index;
+    }
+
+    /// <summary>
+    /// Add a blank sprite layer.
+    /// </summary>
+    public Layer AddBlankLayer(Entity<SpriteComponent> sprite, int? index = null)
+    {
+        var layer = new Layer();
+        AddLayer(sprite!, layer, index);
+        return layer;
+    }
+
+    #endregion
+}

Robust.Client/GameObjects/EntitySystems/SpriteSystem.LayerGetters.cs

@@ -0,0 +1,150 @@
+using System;
+using Robust.Client.Graphics;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Graphics.RSI;
+using static Robust.Client.GameObjects.SpriteComponent;
+using static Robust.Client.Graphics.RSI;
+
+namespace Robust.Client.GameObjects;
+
+// This partial class contains various public methods for reading a layer's properties
+public sealed partial class SpriteSystem
+{
+    #region RsiState
+
+    /// <summary>
+    /// Get the RSI state being used by the current layer. Note that the return value may be an invalid state. E.g.,
+    /// this might be a texture layer that does not use RSIs.
+    /// </summary>
+    public StateId LayerGetRsiState(Entity<SpriteComponent?> sprite, int index)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            return layer.StateId;
+
+        return StateId.Invalid;
+    }
+
+    /// <summary>
+    /// Get the RSI state being used by the current layer. Note that the return value may be an invalid state. E.g.,
+    /// this might be a texture layer that does not use RSIs.
+    /// </summary>
+    public StateId LayerGetRsiState(Entity<SpriteComponent?> sprite, string key, StateId state)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            return layer.StateId;
+
+        return StateId.Invalid;
+    }
+
+    /// <summary>
+    /// Get the RSI state being used by the current layer. Note that the return value may be an invalid state. E.g.,
+    /// this might be a texture layer that does not use RSIs.
+    /// </summary>
+    public StateId LayerGetRsiState(Entity<SpriteComponent?> sprite, Enum key, StateId state)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            return layer.StateId;
+
+        return StateId.Invalid;
+    }
+
+    #endregion
+
+    #region RsiState
+
+    /// <summary>
+    /// Returns the RSI being used by the layer to resolve it's RSI state. If the layer does not specify an RSI, this
+    /// will just be the base RSI of the owning sprite (<see cref="SpriteComponent.BaseRSI"/>).
+    /// </summary>
+    public RSI? LayerGetEffectiveRsi(Entity<SpriteComponent?> sprite, int index)
+    {
+        TryGetLayer(sprite, index, out var layer, true);
+        return layer?.ActualRsi;
+    }
+
+    /// <summary>
+    /// Returns the RSI being used by the layer to resolve it's RSI state. If the layer does not specify an RSI, this
+    /// will just be the base RSI of the owning sprite (<see cref="SpriteComponent.BaseRSI"/>).
+    /// </summary>
+    public RSI? LayerGetEffectiveRsi(Entity<SpriteComponent?> sprite, string key, StateId state)
+    {
+        TryGetLayer(sprite, key, out var layer, true);
+        return layer?.ActualRsi;
+    }
+
+    /// <summary>
+    /// Returns the RSI being used by the layer to resolve it's RSI state. If the layer does not specify an RSI, this
+    /// will just be the base RSI of the owning sprite (<see cref="SpriteComponent.BaseRSI"/>).
+    /// </summary>
+    public RSI? LayerGetEffectiveRsi(Entity<SpriteComponent?> sprite, Enum key, StateId state)
+    {
+        TryGetLayer(sprite, key, out var layer, true);
+        return layer?.ActualRsi;
+    }
+
+    #endregion
+
+    #region Directions
+
+    public RsiDirectionType LayerGetDirections(Entity<SpriteComponent?> sprite, int index)
+    {
+        return TryGetLayer(sprite, index, out var layer, true)
+            ? LayerGetDirections(layer)
+            : RsiDirectionType.Dir1;
+    }
+
+
+    public RsiDirectionType LayerGetDirections(Entity<SpriteComponent?> sprite, Enum key)
+    {
+        return TryGetLayer(sprite, key, out var layer, true)
+            ? LayerGetDirections(layer)
+            : RsiDirectionType.Dir1;
+    }
+
+    public RsiDirectionType LayerGetDirections(Entity<SpriteComponent?> sprite, string key)
+    {
+        return TryGetLayer(sprite, key, out var layer, true)
+            ? LayerGetDirections(layer)
+            : RsiDirectionType.Dir1;
+    }
+
+    public RsiDirectionType LayerGetDirections(Layer layer)
+    {
+        if (!layer.StateId.IsValid)
+            return RsiDirectionType.Dir1;
+
+        // Pull texture from RSI state instead.
+        if (layer.ActualRsi is not {} rsi || !rsi.TryGetState(layer.StateId, out var state))
+            return RsiDirectionType.Dir1;
+
+        return state.RsiDirections;
+    }
+
+    public int LayerGetDirectionCount(Entity<SpriteComponent?> sprite, int index)
+    {
+        return TryGetLayer(sprite, index, out var layer, true) ? LayerGetDirectionCount(layer) : 1;
+    }
+
+    public int LayerGetDirectionCount(Entity<SpriteComponent?> sprite, Enum key)
+    {
+        return TryGetLayer(sprite, key, out var layer, true) ? LayerGetDirectionCount(layer) : 1;
+    }
+
+    public int LayerGetDirectionCount(Entity<SpriteComponent?> sprite, string key)
+    {
+        return TryGetLayer(sprite, key, out var layer, true) ? LayerGetDirectionCount(layer) : 1;
+    }
+
+    public int LayerGetDirectionCount(Layer layer)
+    {
+        return LayerGetDirections(layer) switch
+        {
+            RsiDirectionType.Dir1 => 1,
+            RsiDirectionType.Dir4 => 4,
+            RsiDirectionType.Dir8 => 8,
+            _ => throw new ArgumentOutOfRangeException()
+        };
+    }
+
+    #endregion
+}

Robust.Client/GameObjects/EntitySystems/SpriteSystem.LayerMap.cs

@@ -0,0 +1,299 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Robust.Shared.GameObjects;
+using static Robust.Client.GameObjects.SpriteComponent;
+
+namespace Robust.Client.GameObjects;
+
+// This partial class contains various public methods for manipulating layer mappings.
+public sealed partial class SpriteSystem
+{
+    /// <summary>
+    /// Map an enum to a layer index.
+    /// </summary>
+    public void LayerMapSet(Entity<SpriteComponent?> sprite, Enum key, int index)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (index < 0 || index >= sprite.Comp.Layers.Count)
+            throw new ArgumentOutOfRangeException(nameof(index));
+
+        sprite.Comp.LayerMap[key] = index;
+    }
+
+    /// <summary>
+    /// Map string to a layer index. If possible, it is preferred to use an enum key.
+    /// string keys mainly exist to make it easier to define custom layer keys in yaml.
+    /// </summary>
+    public void LayerMapSet(Entity<SpriteComponent?> sprite, string key, int index)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (index < 0 || index >= sprite.Comp.Layers.Count)
+            throw new ArgumentOutOfRangeException(nameof(index));
+
+        sprite.Comp.LayerMap[key] = index;
+    }
+
+    /// <summary>
+    /// Map an enum to a layer index.
+    /// </summary>
+    public void LayerMapAdd(Entity<SpriteComponent?> sprite, Enum key, int index)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (index < 0 || index >= sprite.Comp.Layers.Count)
+            throw new ArgumentOutOfRangeException(nameof(index));
+
+        sprite.Comp.LayerMap.Add(key, index);
+    }
+
+    /// <summary>
+    /// Map a string to a layer index. If possible, it is preferred to use an enum key.
+    /// string keys mainly exist to make it easier to define custom layer keys in yaml.
+    /// </summary>
+    public void LayerMapAdd(Entity<SpriteComponent?> sprite, string key, int index)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (index < 0 || index >= sprite.Comp.Layers.Count)
+            throw new ArgumentOutOfRangeException(nameof(index));
+
+        sprite.Comp.LayerMap.Add(key, index);
+    }
+
+    /// <summary>
+    /// Remove an enum mapping.
+    /// </summary>
+    public bool LayerMapRemove(Entity<SpriteComponent?> sprite, Enum key)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return false;
+
+        return sprite.Comp.LayerMap.Remove(key);
+    }
+
+    /// <summary>
+    /// Remove a string mapping.
+    /// </summary>
+    public bool LayerMapRemove(Entity<SpriteComponent?> sprite, string key)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return false;
+
+        return sprite.Comp.LayerMap.Remove(key);
+    }
+
+    /// <summary>
+    /// Remove an enum mapping.
+    /// </summary>
+    public bool LayerMapRemove(Entity<SpriteComponent?> sprite, Enum key, out int index)
+    {
+        if (_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return sprite.Comp.LayerMap.Remove(key, out index);
+
+        index = 0;
+        return false;
+    }
+
+    /// <summary>
+    /// Remove a string mapping.
+    /// </summary>
+    public bool LayerMapRemove(Entity<SpriteComponent?> sprite, string key, out int index)
+    {
+        if (_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return sprite.Comp.LayerMap.Remove(key, out index);
+
+        index = 0;
+        return false;
+    }
+
+    /// <summary>
+    /// Attempt to resolve an enum mapping.
+    /// </summary>
+    public bool LayerMapTryGet(Entity<SpriteComponent?> sprite, Enum key, out int index, bool logMissing)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+        {
+            index = 0;
+            return false;
+        }
+
+        if (sprite.Comp.LayerMap.TryGetValue(key, out index))
+            return true;
+
+        if (logMissing)
+            Log.Error($"Layer with key '{key}' does not exist on entity {ToPrettyString(sprite)}! Trace:\n{Environment.StackTrace}");
+
+        return false;
+    }
+
+    /// <summary>
+    /// Attempt to resolve a string mapping.
+    /// </summary>
+    public bool LayerMapTryGet(Entity<SpriteComponent?> sprite, string key, out int index, bool logMissing)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+        {
+            index = 0;
+            return false;
+        }
+
+        if (sprite.Comp.LayerMap.TryGetValue(key, out index))
+            return true;
+
+        if (logMissing)
+            Log.Error($"Layer with key '{key}' does not exist on entity {ToPrettyString(sprite)}! Trace:\n{Environment.StackTrace}");
+
+        return false;
+    }
+
+    /// <summary>
+    /// Attempt to resolve an enum mapping.
+    /// </summary>
+    public bool TryGetLayer(Entity<SpriteComponent?> sprite, Enum key, [NotNullWhen(true)] out Layer? layer, bool logMissing)
+    {
+        layer = null;
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+            return false;
+
+        return LayerMapTryGet(sprite, key, out var index, logMissing)
+               && TryGetLayer(sprite, index, out layer, logMissing);
+    }
+
+    /// <summary>
+    /// Attempt to resolve a string mapping.
+    /// </summary>
+    public bool TryGetLayer(Entity<SpriteComponent?> sprite, string key, [NotNullWhen(true)] out Layer? layer, bool logMissing)
+    {
+        layer = null;
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+            return false;
+
+        return LayerMapTryGet(sprite, key, out var index, logMissing)
+               && TryGetLayer(sprite, index, out layer, logMissing);
+    }
+
+    public int LayerMapGet(Entity<SpriteComponent?> sprite, Enum key)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return -1;
+
+        return sprite.Comp.LayerMap[key];
+    }
+
+    public int LayerMapGet(Entity<SpriteComponent?> sprite, string key)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return -1;
+
+        return sprite.Comp.LayerMap[key];
+    }
+
+    public bool LayerExists(Entity<SpriteComponent?> sprite, string key)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return false;
+
+        return sprite.Comp.LayerMap.TryGetValue(key, out var index)
+               && LayerExists(sprite, index);
+    }
+
+    public bool LayerExists(Entity<SpriteComponent?> sprite, Enum key)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return false;
+
+        return sprite.Comp.LayerMap.TryGetValue(key, out var index)
+               && LayerExists(sprite, index);
+    }
+
+    /// <summary>
+    /// Ensures that a layer with the given key exists and return the layer's index.
+    /// If the layer does not yet exist, this will create and add a blank layer.
+    /// </summary>
+    public int LayerMapReserve(Entity<SpriteComponent?> sprite, Enum key)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return -1;
+
+        if (LayerMapTryGet(sprite, key, out var layerIndex, false))
+            return layerIndex;
+
+        var layer = AddBlankLayer(sprite!);
+        LayerMapSet(sprite, key, layer.Index);
+        return layer.Index;
+    }
+
+    /// <inheritdoc cref="LayerMapReserve(Entity{SpriteComponent?},System.Enum)"/>
+    public int LayerMapReserve(Entity<SpriteComponent?> sprite, string key)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return -1;
+
+        if (LayerMapTryGet(sprite, key, out var layerIndex, false))
+            return layerIndex;
+
+        var layer = AddBlankLayer(sprite!);
+        LayerMapSet(sprite, key, layer.Index);
+        return layer.Index;
+    }
+
+    public bool RemoveLayer(Entity<SpriteComponent?> sprite, string key, bool logMissing = true)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+            return false;
+
+        if (!LayerMapTryGet(sprite, key, out var index, logMissing))
+            return false;
+
+        return RemoveLayer(sprite, index, logMissing);
+    }
+
+    public bool RemoveLayer(Entity<SpriteComponent?> sprite, Enum key, bool logMissing = true)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+            return false;
+
+        if (!LayerMapTryGet(sprite, key, out var index, logMissing))
+            return false;
+
+        return RemoveLayer(sprite, index, logMissing);
+    }
+
+    public bool RemoveLayer(
+        Entity<SpriteComponent?> sprite,
+        string key,
+        [NotNullWhen(true)] out Layer? layer,
+        bool logMissing = true)
+    {
+        layer = null;
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+            return false;
+
+        if (!LayerMapTryGet(sprite, key, out var index, logMissing))
+            return false;
+
+        return RemoveLayer(sprite, index, out layer, logMissing);
+    }
+
+    public bool RemoveLayer(
+        Entity<SpriteComponent?> sprite,
+        Enum key,
+        [NotNullWhen(true)] out Layer? layer,
+        bool logMissing = true)
+    {
+        layer = null;
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp, logMissing))
+            return false;
+
+        if (!LayerMapTryGet(sprite, key, out var index, logMissing))
+            return false;
+
+        return RemoveLayer(sprite, index, out layer, logMissing);
+    }
+}

Robust.Client/GameObjects/EntitySystems/SpriteSystem.LayerSetters.cs

@@ -0,0 +1,605 @@
+using System;
+using System.Numerics;
+using Robust.Client.Graphics;
+using Robust.Client.ResourceManagement;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Maths;
+using Robust.Shared.Utility;
+using static Robust.Client.GameObjects.SpriteComponent;
+using static Robust.Client.Graphics.RSI;
+
+#pragma warning disable CS0618 // Type or member is obsolete
+
+namespace Robust.Client.GameObjects;
+
+// This partial class contains various public methods for modifying a layer's properties.
+public sealed partial class SpriteSystem
+{
+    #region SetData
+
+    public void LayerSetData(Entity<SpriteComponent?> sprite, int index, PrototypeLayerData data)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetData(layer, data);
+    }
+
+    public void LayerSetData(Entity<SpriteComponent?> sprite, string key, PrototypeLayerData data)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetData(layer, data);
+    }
+
+    public void LayerSetData(Entity<SpriteComponent?> sprite, Enum key, PrototypeLayerData data)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetData(layer, data);
+    }
+
+    public void LayerSetData(Layer layer, PrototypeLayerData data)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+        // TODO SPRITE ECS
+        layer._parent.LayerSetData(layer, data);
+    }
+
+    #endregion
+
+    #region SpriteSpecifier
+
+    public void LayerSetSprite(Entity<SpriteComponent?> sprite, int index, SpriteSpecifier specifier)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetSprite(layer, specifier);
+    }
+
+    public void LayerSetSprite(Entity<SpriteComponent?> sprite, string key, SpriteSpecifier specifier)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetSprite(layer, specifier);
+    }
+
+    public void LayerSetSprite(Entity<SpriteComponent?> sprite, Enum key, SpriteSpecifier specifier)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetSprite(layer, specifier);
+    }
+
+    public void LayerSetSprite(Layer layer, SpriteSpecifier specifier)
+    {
+        switch (specifier)
+        {
+            case SpriteSpecifier.Texture tex:
+                LayerSetTexture(layer, tex.TexturePath);
+                break;
+
+            case SpriteSpecifier.Rsi rsi:
+                LayerSetRsi(layer, rsi.RsiPath, rsi.RsiState);
+                break;
+
+            default:
+                throw new NotImplementedException();
+        }
+    }
+
+    #endregion
+
+    #region Texture
+
+    public void LayerSetTexture(Entity<SpriteComponent?> sprite, int index, Texture? texture)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetTexture(layer, texture);
+    }
+
+    public void LayerSetTexture(Entity<SpriteComponent?> sprite, string key, Texture? texture)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetTexture(layer, texture);
+    }
+
+    public void LayerSetTexture(Entity<SpriteComponent?> sprite, Enum key, Texture? texture)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetTexture(layer, texture);
+    }
+
+    public void LayerSetTexture(Layer layer, Texture? texture)
+    {
+        LayerSetRsiState(layer, StateId.Invalid, refresh: true);
+        layer.Texture = texture;
+    }
+
+    public void LayerSetTexture(Entity<SpriteComponent?> sprite, int index, ResPath path)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetTexture(layer, path);
+    }
+
+    public void LayerSetTexture(Entity<SpriteComponent?> sprite, string key, ResPath path)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetTexture(layer, path);
+    }
+
+    public void LayerSetTexture(Entity<SpriteComponent?> sprite, Enum key, ResPath path)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetTexture(layer, path);
+    }
+
+    private void LayerSetTexture(Layer layer, ResPath path)
+    {
+        if (!_resourceCache.TryGetResource<TextureResource>(TextureRoot / path, out var texture))
+        {
+            if (path.Extension == "rsi")
+                Log.Error($"Expected texture but got rsi '{path}', did you mean 'sprite:' instead of 'texture:'?");
+            Log.Error($"Unable to load texture '{path}'. Trace:\n{Environment.StackTrace}");
+        }
+
+        LayerSetTexture(layer, texture?.Texture);
+    }
+
+    #endregion
+
+    #region RsiState
+
+    public void LayerSetRsiState(Entity<SpriteComponent?> sprite, int index, StateId state)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetRsiState(layer, state);
+    }
+
+    public void LayerSetRsiState(Entity<SpriteComponent?> sprite, string key, StateId state)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRsiState(layer, state);
+    }
+
+    public void LayerSetRsiState(Entity<SpriteComponent?> sprite, Enum key, StateId state)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRsiState(layer, state);
+    }
+
+    public void LayerSetRsiState(Layer layer, StateId state, bool refresh = false)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        if (layer.StateId == state && !refresh)
+            return;
+
+        layer.StateId = state;
+        RefreshCachedState(layer, true, null);
+        _tree.QueueTreeUpdate(layer.Owner);
+        QueueUpdateIsInert(layer.Owner);
+        layer.BoundsDirty = true;
+        layer.Owner.Comp.BoundsDirty = true;
+    }
+
+    #endregion
+
+    #region Rsi
+
+    public void LayerSetRsi(Entity<SpriteComponent?> sprite, int index, RSI? rsi, StateId? state = null)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetRsi(layer, rsi, state);
+    }
+
+    public void LayerSetRsi(Entity<SpriteComponent?> sprite, string key, RSI? rsi, StateId? state = null)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRsi(layer, rsi, state);
+    }
+
+    public void LayerSetRsi(Entity<SpriteComponent?> sprite, Enum key, RSI? rsi, StateId? state = null)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRsi(layer, rsi, state);
+    }
+
+    public void LayerSetRsi(Layer layer, RSI? rsi, StateId? state = null)
+    {
+        layer._rsi = rsi;
+        LayerSetRsiState(layer, state ?? layer.StateId, refresh: true);
+    }
+
+    public void LayerSetRsi(Entity<SpriteComponent?> sprite, int index, ResPath rsi, StateId? state = null)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetRsi(layer, rsi, state);
+    }
+
+    public void LayerSetRsi(Entity<SpriteComponent?> sprite, string key, ResPath rsi, StateId? state = null)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRsi(layer, rsi, state);
+    }
+
+    public void LayerSetRsi(Entity<SpriteComponent?> sprite, Enum key, ResPath rsi, StateId? state = null)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRsi(layer, rsi, state);
+    }
+
+    public void LayerSetRsi(Layer layer, ResPath rsi, StateId? state = null)
+    {
+        if (!_resourceCache.TryGetResource<RSIResource>(TextureRoot / rsi, out var res))
+            Log.Error($"Unable to load RSI '{rsi}' for entity {ToPrettyString(layer.Owner)}. Trace:\n{Environment.StackTrace}");
+
+        LayerSetRsi(layer, res?.RSI, state);
+    }
+
+    #endregion
+
+    #region Scale
+
+    public void LayerSetScale(Entity<SpriteComponent?> sprite, int index, Vector2 value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetScale(layer, value);
+    }
+
+    public void LayerSetScale(Entity<SpriteComponent?> sprite, string key, Vector2 value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetScale(layer, value);
+    }
+
+    public void LayerSetScale(Entity<SpriteComponent?> sprite, Enum key, Vector2 value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetScale(layer, value);
+    }
+
+    public void LayerSetScale(Layer layer, Vector2 value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        if (layer._scale.EqualsApprox(value))
+            return;
+
+        if (!ValidateScale(layer.Owner, value))
+            return;
+
+        layer._scale = value;
+        layer.UpdateLocalMatrix();
+        _tree.QueueTreeUpdate(layer.Owner);
+        layer.BoundsDirty = true;
+        layer.Owner.Comp.BoundsDirty = true;
+    }
+
+    #endregion
+
+    #region Rotation
+
+    public void LayerSetRotation(Entity<SpriteComponent?> sprite, int index, Angle value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetRotation(layer, value);
+    }
+
+    public void LayerSetRotation(Entity<SpriteComponent?> sprite, string key, Angle value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRotation(layer, value);
+    }
+
+    public void LayerSetRotation(Entity<SpriteComponent?> sprite, Enum key, Angle value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRotation(layer, value);
+    }
+
+    public void LayerSetRotation(Layer layer, Angle value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        if (layer._rotation.EqualsApprox(value))
+            return;
+
+        layer._rotation = value;
+        layer.UpdateLocalMatrix();
+        _tree.QueueTreeUpdate(layer.Owner);
+        layer.BoundsDirty = true;
+        layer.Owner.Comp.BoundsDirty = true;
+    }
+
+    #endregion
+
+    #region Offset
+
+    public void LayerSetOffset(Entity<SpriteComponent?> sprite, int index, Vector2 value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetOffset(layer, value);
+    }
+
+    public void LayerSetOffset(Entity<SpriteComponent?> sprite, string key, Vector2 value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetOffset(layer, value);
+    }
+
+    public void LayerSetOffset(Entity<SpriteComponent?> sprite, Enum key, Vector2 value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetOffset(layer, value);
+    }
+
+    public void LayerSetOffset(Layer layer, Vector2 value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        if (layer._offset.EqualsApprox(value))
+            return;
+
+        layer._offset = value;
+        layer.UpdateLocalMatrix();
+        _tree.QueueTreeUpdate(layer.Owner);
+        layer.BoundsDirty = true;
+        layer.Owner.Comp.BoundsDirty = true;
+    }
+
+    #endregion
+
+    #region Visible
+
+    public void LayerSetVisible(Entity<SpriteComponent?> sprite, int index, bool value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetVisible(layer, value);
+    }
+
+    public void LayerSetVisible(Entity<SpriteComponent?> sprite, string key, bool value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetVisible(layer, value);
+    }
+
+    public void LayerSetVisible(Entity<SpriteComponent?> sprite, Enum key, bool value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetVisible(layer, value);
+    }
+
+    public void LayerSetVisible(Layer layer, bool value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        if (layer._visible == value)
+            return;
+
+        layer._visible = value;
+        QueueUpdateIsInert(layer.Owner);
+        _tree.QueueTreeUpdate(layer.Owner);
+        layer.Owner.Comp.BoundsDirty = true;
+    }
+
+    #endregion
+
+    #region Color
+
+    public void LayerSetColor(Entity<SpriteComponent?> sprite, int index, Color value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetColor(layer, value);
+    }
+
+    public void LayerSetColor(Entity<SpriteComponent?> sprite, string key, Color value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetColor(layer, value);
+    }
+
+    public void LayerSetColor(Entity<SpriteComponent?> sprite, Enum key, Color value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetColor(layer, value);
+    }
+
+    public void LayerSetColor(Layer layer, Color value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        layer.Color = value;
+    }
+
+    #endregion
+
+    #region DirOffset
+
+    public void LayerSetDirOffset(Entity<SpriteComponent?> sprite, int index, DirectionOffset value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetDirOffset(layer, value);
+    }
+
+    public void LayerSetDirOffset(Entity<SpriteComponent?> sprite, string key, DirectionOffset value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetDirOffset(layer, value);
+    }
+
+    public void LayerSetDirOffset(Entity<SpriteComponent?> sprite, Enum key, DirectionOffset value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetDirOffset(layer, value);
+    }
+
+    public void LayerSetDirOffset(Layer layer, DirectionOffset value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        layer.DirOffset = value;
+    }
+
+    #endregion
+
+    #region AnimationTime
+
+    public void LayerSetAnimationTime(Entity<SpriteComponent?> sprite, int index, float value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetAnimationTime(layer, value);
+    }
+
+    public void LayerSetAnimationTime(Entity<SpriteComponent?> sprite, string key, float value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetAnimationTime(layer, value);
+    }
+
+    public void LayerSetAnimationTime(Entity<SpriteComponent?> sprite, Enum key, float value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetAnimationTime(layer, value);
+    }
+
+    public void LayerSetAnimationTime(Layer layer, float value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        if (!layer.StateId.IsValid)
+            return;
+
+        if (layer.ActualRsi is not { } rsi)
+            return;
+
+        var state = rsi[layer.StateId];
+        if (value > layer.AnimationTime)
+        {
+            // Handle advancing differently from going backwards.
+            layer.AnimationTimeLeft -= (value - layer.AnimationTime);
+        }
+        else
+        {
+            // Going backwards we re-calculate from zero.
+            // Definitely possible to optimize this for going backwards but I'm too lazy to figure that out.
+            layer.AnimationTimeLeft = -value + state.GetDelay(0);
+            layer.AnimationFrame = 0;
+        }
+
+        layer.AnimationTime = value;
+        layer.AdvanceFrameAnimation(state);
+        layer.SetAnimationTime(value);
+    }
+
+    #endregion
+
+    #region AutoAnimated
+
+    public void LayerSetAutoAnimated(Entity<SpriteComponent?> sprite, int index, bool value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetAutoAnimated(layer, value);
+    }
+
+    public void LayerSetAutoAnimated(Entity<SpriteComponent?> sprite, string key, bool value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetAutoAnimated(layer, value);
+    }
+
+    public void LayerSetAutoAnimated(Entity<SpriteComponent?> sprite, Enum key, bool value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetAutoAnimated(layer, value);
+    }
+
+    public void LayerSetAutoAnimated(Layer layer, bool value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        if (layer._autoAnimated == value)
+            return;
+
+        layer._autoAnimated = value;
+        QueueUpdateIsInert(layer.Owner);
+    }
+
+    #endregion
+
+    #region LayerSetRenderingStrategy
+
+    public void LayerSetRenderingStrategy(Entity<SpriteComponent?> sprite, int index, LayerRenderingStrategy value)
+    {
+        if (TryGetLayer(sprite, index, out var layer, true))
+            LayerSetRenderingStrategy(layer, value);
+    }
+
+    public void LayerSetRenderingStrategy(Entity<SpriteComponent?> sprite, string key, LayerRenderingStrategy value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRenderingStrategy(layer, value);
+    }
+
+    public void LayerSetRenderingStrategy(Entity<SpriteComponent?> sprite, Enum key, LayerRenderingStrategy value)
+    {
+        if (TryGetLayer(sprite, key, out var layer, true))
+            LayerSetRenderingStrategy(layer, value);
+    }
+
+    public void LayerSetRenderingStrategy(Layer layer, LayerRenderingStrategy value)
+    {
+        DebugTools.Assert(layer.Owner != default);
+        DebugTools.AssertNotNull(layer.Owner.Comp);
+        DebugTools.AssertEqual(layer.Owner.Comp.Layers[layer.Index], layer);
+
+        layer.RenderingStrategy = value;
+        layer.BoundsDirty = true;
+        layer.Owner.Comp.BoundsDirty = true;
+        _tree.QueueTreeUpdate(layer.Owner);
+    }
+
+    #endregion
+
+    /// <summary>
+    /// Refreshes an RSI layer's cached RSI state.
+    /// </summary>
+    private void RefreshCachedState(Layer layer, bool logErrors, RSI.State? fallback)
+    {
+        if (!layer.StateId.IsValid)
+        {
+            layer._actualState = null;
+        }
+        else if (layer.ActualRsi is not { } rsi)
+        {
+            layer._actualState = fallback ?? GetFallbackState();
+            if (logErrors)
+                Log.Error(
+                    $"{ToPrettyString(layer.Owner)} has no RSI to pull new state from! Trace:\n{Environment.StackTrace}");
+        }
+        else if (!rsi.TryGetState(layer.StateId, out layer._actualState))
+        {
+            layer._actualState = fallback ?? GetFallbackState();
+            if (logErrors)
+                Log.Error(
+                    $"{ToPrettyString(layer.Owner)}'s state '{layer.StateId}' does not exist in RSI {rsi.Path}. Trace:\n{Environment.StackTrace}");
+        }
+
+        layer.AnimationFrame = 0;
+        layer.AnimationTime = 0;
+        layer.AnimationTimeLeft = layer._actualState?.GetDelay(0) ?? 0f;
+    }
+}

Robust.Client/GameObjects/EntitySystems/SpriteSystem.Render.cs

@@ -0,0 +1,196 @@
+using System.Numerics;
+using Robust.Client.Graphics;
+using Robust.Client.Graphics.Clyde;
+using Robust.Client.Utility;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Graphics.RSI;
+using Robust.Shared.Maths;
+using Robust.Shared.Utility;
+using static Robust.Client.GameObjects.SpriteComponent;
+using Vector4 = Robust.Shared.Maths.Vector4;
+using SysVec4 = System.Numerics.Vector4;
+
+namespace Robust.Client.GameObjects;
+
+// This partial class contains code related to actually rendering sprites.
+public sealed partial class SpriteSystem
+{
+    public void RenderSprite(
+        Entity<SpriteComponent> sprite,
+        DrawingHandleWorld drawingHandle,
+        Angle eyeRotation,
+        Angle worldRotation,
+        Vector2 worldPosition)
+    {
+        RenderSprite(sprite,
+            drawingHandle,
+            eyeRotation,
+            worldRotation,
+            worldPosition,
+            sprite.Comp.EnableDirectionOverride ? sprite.Comp.DirectionOverride : null);
+    }
+
+    public void RenderSprite(
+        Entity<SpriteComponent> sprite,
+        DrawingHandleWorld drawingHandle,
+        Angle eyeRotation,
+        Angle worldRotation,
+        Vector2 worldPosition,
+        Direction? overrideDirection)
+    {
+        // TODO SPRITE RENDERING
+        // Add fast path for simple sprites.
+        // I.e., when a sprite is modified, check if it is "simple". If it is. cache texture information in a struct
+        // and use a fast path here.
+        // E.g., simple 1-directional, 1-layer sprites can basically become a direct texture draw call. (most in game items).
+        // Similarly, 1-directional multi-layer sprites can become a sequence of direct draw calls (most in game walls).
+
+        if (!sprite.Comp.IsInert)
+            _queuedFrameUpdate.Add(sprite);
+
+        var angle = worldRotation + eyeRotation; // angle on-screen. Used to decide the direction of 4/8 directional RSIs
+        angle = angle.Reduced().FlipPositive();  // Reduce the angles to fix math shenanigans
+
+        var cardinal = Angle.Zero;
+
+        // If we have a 1-directional sprite then snap it to try and always face it south if applicable.
+        if (sprite.Comp is {NoRotation: false, SnapCardinals: true})
+            cardinal = angle.RoundToCardinalAngle();
+
+        // worldRotation + eyeRotation should be the angle of the entity on-screen. If no-rot is enabled this is just set to zero.
+        // However, at some point later the eye-matrix is applied separately, so we subtract -eye rotation for now:
+        var entityMatrix = Matrix3Helpers.CreateTransform(worldPosition, sprite.Comp.NoRotation ? -eyeRotation : worldRotation - cardinal);
+        var spriteMatrix = Matrix3x2.Multiply(sprite.Comp.LocalMatrix, entityMatrix);
+
+        // Fast path for when all sprites use the same transform matrix
+        if (!sprite.Comp.GranularLayersRendering)
+        {
+            foreach (var layer in sprite.Comp.Layers)
+            {
+                RenderLayer(layer, drawingHandle, ref spriteMatrix, angle, overrideDirection);
+            }
+            return;
+        }
+
+        //Default rendering (NoRotation = false)
+        entityMatrix = Matrix3Helpers.CreateTransform(worldPosition, worldRotation);
+        var transformDefault = Matrix3x2.Multiply(sprite.Comp.LocalMatrix, entityMatrix);
+
+        //Snap to cardinals
+        entityMatrix = Matrix3Helpers.CreateTransform(worldPosition, worldRotation - angle.RoundToCardinalAngle());
+        var transformSnap = Matrix3x2.Multiply(sprite.Comp.LocalMatrix, entityMatrix);
+
+        //No rotation
+        entityMatrix = Matrix3Helpers.CreateTransform(worldPosition, -eyeRotation);
+        var transformNoRot = Matrix3x2.Multiply(sprite.Comp.LocalMatrix, entityMatrix);
+
+        foreach (var layer in sprite.Comp.Layers)
+        {
+            switch (layer.RenderingStrategy)
+            {
+                case LayerRenderingStrategy.UseSpriteStrategy:
+                    RenderLayer(layer, drawingHandle, ref spriteMatrix, angle, overrideDirection);
+                    break;
+                case LayerRenderingStrategy.Default:
+                    RenderLayer(layer, drawingHandle, ref transformDefault, angle, overrideDirection);
+                    break;
+                case LayerRenderingStrategy.NoRotation:
+                    RenderLayer(layer, drawingHandle, ref transformNoRot, angle, overrideDirection);
+                    break;
+                case LayerRenderingStrategy.SnapToCardinals:
+                    RenderLayer(layer, drawingHandle, ref transformSnap, angle, overrideDirection);
+                    break;
+                default:
+                    Log.Error($"Tried to render a layer with unknown rendering stragegy: {layer.RenderingStrategy}");
+                    break;
+            }
+        }
+    }
+
+    /// <summary>
+    /// Render a layer. This assumes that the input angle is between 0 and 2pi.
+    /// </summary>
+    private void RenderLayer(Layer layer, DrawingHandleWorld drawingHandle, ref Matrix3x2 spriteMatrix, Angle angle, Direction? overrideDirection)
+    {
+        if (!layer.Visible || layer.Blank)
+            return;
+
+        var state = layer._actualState;
+        var dir = state == null ? RsiDirection.South : Layer.GetDirection(state.RsiDirections, angle);
+
+        // Set the drawing transform for this layer
+        layer.GetLayerDrawMatrix(dir, out var layerMatrix, layer.Owner.Comp.NoRotation);
+
+        // The direction used to draw the sprite can differ from the one that the angle would naively suggest,
+        // due to direction overrides or offsets.
+        if (overrideDirection != null && state != null)
+            dir = overrideDirection.Value.Convert(state.RsiDirections);
+        dir = dir.OffsetRsiDir(layer.DirOffset);
+
+        var texture = state?.GetFrame(dir, layer.AnimationFrame) ?? layer.Texture ?? GetFallbackTexture();
+
+        // TODO SPRITE
+        // Refactor shader-param-layers to a separate layer type after layers are split into types & collections.
+        // I.e., separate Layer -> RsiLayer, TextureLayer, LayerCollection, SpriteLayer, and ShaderLayer
+        if (layer.CopyToShaderParameters != null)
+        {
+            HandleShaderLayer(layer, texture, layer.CopyToShaderParameters);
+            return;
+        }
+
+        // Set the drawing transform for this layer
+        var transformMatrix = Matrix3x2.Multiply(layerMatrix, spriteMatrix);
+        drawingHandle.SetTransform(in transformMatrix);
+
+        if (layer.Shader != null)
+            drawingHandle.UseShader(layer.Shader);
+
+        var layerColor = layer.Owner.Comp.color * layer.Color;
+        var textureSize = texture.Size / (float) EyeManager.PixelsPerMeter;
+        var quad = Box2.FromDimensions(textureSize / -2, textureSize);
+
+        if (layer.UnShaded)
+        {
+            DebugTools.AssertNull(layer.Shader);
+            DebugTools.Assert(layerColor is {R: >= 0, G: >= 0, B: >= 0, A: >= 0}, "Default shader should not be used with negative color modulation.");
+
+            // Negative color modulation values are by the default shader to disable light shading.
+            // Specifically we set colour = - 1 - colour
+            // This is good enough to ensure that non-negative values become negative & is trivially invertible.
+            layerColor = new(new SysVec4(-1) - layerColor.RGBA);
+        }
+
+        drawingHandle.DrawTextureRectRegion(texture, quad, layerColor);
+
+        if (layer.Shader != null)
+            drawingHandle.UseShader(null);
+    }
+
+    /// <summary>
+    /// Handle a a "fake layer" that just exists to modify the parameters of a shader being used by some other
+    /// layer.
+    /// </summary>
+    private void HandleShaderLayer(Layer layer, Texture texture, CopyToShaderParameters @params)
+    {
+        // Multiple atrocities to god being committed right here.
+        var otherLayerIdx = layer._parent.LayerMap[@params.LayerKey!];
+        var otherLayer = layer._parent.Layers[otherLayerIdx];
+        if (otherLayer.Shader is not { } shader)
+            return;
+
+        if (!shader.Mutable)
+            otherLayer.Shader = shader = shader.Duplicate();
+
+        var clydeTexture = Clyde.RenderHandle.ExtractTexture(texture, null, out var csr);
+
+        if (@params.ParameterTexture is { } paramTexture)
+            shader.SetParameter(paramTexture, clydeTexture);
+
+        if (@params.ParameterUV is not { } paramUV)
+            return;
+
+        var sr = Clyde.RenderHandle.WorldTextureBoundsToUV(clydeTexture, csr);
+        var uv = new Vector4(sr.Left, sr.Bottom, sr.Right, sr.Top);
+        shader.SetParameter(paramUV, uv);
+    }
+}

Robust.Client/GameObjects/EntitySystems/SpriteSystem.Setters.cs

@@ -0,0 +1,166 @@
+using System;
+using System.Numerics;
+using Robust.Client.Graphics;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Maths;
+
+namespace Robust.Client.GameObjects;
+
+// This partial class contains various public methods for setting sprite component data.
+public sealed partial class SpriteSystem
+{
+    private bool ValidateScale(Entity<SpriteComponent> sprite, Vector2 scale)
+    {
+        if (!(MathF.Abs(scale.X) < 0.005f) && !(MathF.Abs(scale.Y) < 0.005f))
+            return true;
+
+        // Scales of ~0.0025 or lower can lead to singular matrices due to rounding errors.
+        Log.Error(
+            $"Attempted to set layer sprite scale to very small values. Entity: {ToPrettyString(sprite)}. Scale: {scale}");
+
+        return false;
+    }
+
+    #region Transform
+    public void SetScale(Entity<SpriteComponent?> sprite, Vector2 value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (!ValidateScale(sprite!, value))
+            return;
+
+        sprite.Comp._bounds = sprite.Comp._bounds.Scale(value / sprite.Comp.scale);
+        sprite.Comp.scale = value;
+        sprite.Comp.LocalMatrix = Matrix3Helpers.CreateTransform(
+            in sprite.Comp.offset,
+            in sprite.Comp.rotation,
+            in sprite.Comp.scale);
+    }
+
+    public void SetRotation(Entity<SpriteComponent?> sprite, Angle value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        sprite.Comp.rotation = value;
+        sprite.Comp.LocalMatrix = Matrix3Helpers.CreateTransform(
+            in sprite.Comp.offset,
+            in sprite.Comp.rotation,
+            in sprite.Comp.scale);
+    }
+
+    public void SetOffset(Entity<SpriteComponent?> sprite, Vector2 value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        sprite.Comp.offset = value;
+        sprite.Comp.LocalMatrix = Matrix3Helpers.CreateTransform(
+            in sprite.Comp.offset,
+            in sprite.Comp.rotation,
+            in sprite.Comp.scale);
+    }
+    #endregion
+
+    public void SetVisible(Entity<SpriteComponent?> sprite, bool value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (sprite.Comp.Visible == value)
+            return;
+
+        sprite.Comp._visible = value;
+        _tree.QueueTreeUpdate(sprite!);
+    }
+
+    public void SetDrawDepth(Entity<SpriteComponent?> sprite, int value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        sprite.Comp.drawDepth = value;
+    }
+
+    public void SetColor(Entity<SpriteComponent?> sprite, Color value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        sprite.Comp.color = value;
+    }
+
+    /// <summary>
+    /// Modify a sprites base RSI. This is the RSI that is used by any RSI layers that do not specify their own.
+    /// Note that changing the base RSI may result in existing layers having an invalid state. This will not log errors
+    /// under the assumption that the states of each layers will be updated after the base RSI has changed.
+    /// </summary>
+    public void SetBaseRsi(Entity<SpriteComponent?> sprite, RSI? value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (value == sprite.Comp._baseRsi)
+            return;
+
+        sprite.Comp._baseRsi = value;
+        if (value == null)
+            return;
+
+        var fallback = GetFallbackState();
+        for (var i = 0; i < sprite.Comp.Layers.Count; i++)
+        {
+            var layer = sprite.Comp.Layers[i];
+            if (!layer.State.IsValid || layer.RSI != null)
+                continue;
+
+            RefreshCachedState(layer, logErrors: false, fallback);
+
+            if (value.TryGetState(layer.State, out var state))
+            {
+                layer.AnimationTimeLeft = state.GetDelay(0);
+            }
+            else
+            {
+                Log.Error($"Layer {i} no longer has state '{layer.State}' due to base RSI change. Trace:\n{Environment.StackTrace}");
+                layer.Texture = null;
+            }
+        }
+    }
+
+    public void SetContainerOccluded(Entity<SpriteComponent?> sprite, bool value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        sprite.Comp._containerOccluded = value;
+        _tree.QueueTreeUpdate(sprite!);
+    }
+
+    public void SetSnapCardinals(Entity<SpriteComponent?> sprite, bool value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (value == sprite.Comp._snapCardinals)
+            return;
+
+        sprite.Comp._snapCardinals = value;
+        _tree.QueueTreeUpdate(sprite!);
+        DirtyBounds(sprite!);
+    }
+
+    public void SetGranularLayersRendering(Entity<SpriteComponent?> sprite, bool value)
+    {
+        if (!_query.Resolve(sprite.Owner, ref sprite.Comp))
+            return;
+
+        if (value == sprite.Comp.GranularLayersRendering)
+            return;
+
+        sprite.Comp.GranularLayersRendering = value;
+        _tree.QueueTreeUpdate(sprite!);
+        DirtyBounds(sprite!);
+    }
+}

Robust.Client/GameObjects/EntitySystems/SpriteSystem.Specifier.cs

@@ -1,137 +0,0 @@
-using System;
-using System.Collections.Generic;
-using JetBrains.Annotations;
-using Robust.Client.Graphics;
-using Robust.Client.ResourceManagement;
-using Robust.Client.Utility;
-using Robust.Shared.Graphics;
-using Robust.Shared.Map;
-using Robust.Shared.Prototypes;
-using Robust.Shared.Serialization.TypeSerializers.Implementations;
-using Robust.Shared.Utility;
-
-namespace Robust.Client.GameObjects;
-
-public sealed partial class SpriteSystem
-{
-    private readonly Dictionary<string, IRsiStateLike> _cachedPrototypeIcons = new();
-
-    public Texture Frame0(EntityPrototype prototype)
-    {
-        return GetPrototypeIcon(prototype).Default;
-    }
-
-    public Texture Frame0(SpriteSpecifier specifier)
-    {
-        return RsiStateLike(specifier).Default;
-    }
-
-    public IRsiStateLike RsiStateLike(SpriteSpecifier specifier)
-    {
-        switch (specifier)
-        {
-            case SpriteSpecifier.Texture tex:
-                return tex.GetTexture(_resourceCache);
-
-            case SpriteSpecifier.Rsi rsi:
-                return GetState(rsi);
-
-            case SpriteSpecifier.EntityPrototype prototypeIcon:
-                return GetPrototypeIcon(prototypeIcon.EntityPrototypeId);
-
-            default:
-                throw new NotSupportedException();
-        }
-    }
-
-    public Texture GetIcon(IconComponent icon)
-    {
-        return GetState(icon.Icon).Frame0;
-    }
-
-    /// <summary>
-    ///     Returns an icon for a given <see cref="EntityPrototype"/> ID, or a fallback in case of an error.
-    ///     This method caches the result based on the prototype identifier.
-    /// </summary>
-    public IRsiStateLike GetPrototypeIcon(string prototype)
-    {
-        // Check if this prototype has been cached before, and if so return the result.
-        if (_cachedPrototypeIcons.TryGetValue(prototype, out var cachedResult))
-            return cachedResult;
-
-        if (!_proto.TryIndex<EntityPrototype>(prototype, out var entityPrototype))
-        {
-            // The specified prototype doesn't exist, return the fallback "error" sprite.
-            _sawmill.Error("Failed to load PrototypeIcon {0}", prototype);
-            return GetFallbackState();
-        }
-
-        // Generate the icon and cache it in case it's ever needed again.
-        var result = GetPrototypeIcon(entityPrototype);
-        _cachedPrototypeIcons[prototype] = result;
-
-        return result;
-    }
-
-    /// <summary>
-    ///     Returns an icon for a given <see cref="EntityPrototype"/> ID, or a fallback in case of an error.
-    ///     This method does NOT cache the result.
-    /// </summary>
-    public IRsiStateLike GetPrototypeIcon(EntityPrototype prototype)
-    {
-        // IconComponent takes precedence. If it has a valid icon, return that. Otherwise, continue as normal.
-        if (prototype.Components.TryGetValue("Icon", out var compData)
-            && compData.Component is IconComponent icon)
-        {
-            return GetIcon(icon);
-        }
-
-        // If the prototype doesn't have a SpriteComponent, then there's nothing we can do but return the fallback.
-        if (!prototype.Components.ContainsKey("Sprite"))
-        {
-            return GetFallbackState();
-        }
-
-        // Finally, we use spawn a dummy entity to get its icon.
-        var dummy = Spawn(prototype.ID, MapCoordinates.Nullspace);
-        var spriteComponent = EnsureComp<SpriteComponent>(dummy);
-        var result = spriteComponent.Icon ?? GetFallbackState();
-        Del(dummy);
-
-        return result;
-    }
-
-    [Pure]
-    public RSI.State GetFallbackState()
-    {
-        return _resourceCache.GetFallback<RSIResource>().RSI["error"];
-    }
-
-    [Pure]
-    public RSI.State GetState(SpriteSpecifier.Rsi rsiSpecifier)
-    {
-        if (_resourceCache.TryGetResource<RSIResource>(
-                SpriteSpecifierSerializer.TextureRoot / rsiSpecifier.RsiPath,
-                out var theRsi) &&
-            theRsi.RSI.TryGetState(rsiSpecifier.RsiState, out var state))
-        {
-            return state;
-        }
-
-        _sawmill.Error("Failed to load RSI {0}", rsiSpecifier.RsiPath);
-        return GetFallbackState();
-    }
-
-    private void OnPrototypesReloaded(PrototypesReloadedEventArgs args)
-    {
-        if (!args.TryGetModified<EntityPrototype>(out var modified))
-            return;
-
-        // Remove all changed prototypes from the cache, if they're there.
-        foreach (var prototype in modified)
-        {
-            // Let's be lazy and not regenerate them until something needs them again.
-            _cachedPrototypeIcons.Remove(prototype);
-        }
-    }
-}

Robust.Client/GameObjects/EntitySystems/SpriteSystem.cs

@@ -13,10 +13,9 @@ using Robust.Shared.GameObjects;
 using Robust.Shared.Graphics.RSI;
 using Robust.Shared.IoC;
 using Robust.Shared.Log;
-using Robust.Shared.Map;
 using Robust.Shared.Maths;
-using Robust.Shared.Physics;
 using Robust.Shared.Prototypes;
+using Robust.Shared.Serialization.TypeSerializers.Implementations;
 using Robust.Shared.Timing;
 using Robust.Shared.Utility;
 using static Robust.Client.GameObjects.SpriteComponent;
@@ -36,24 +35,20 @@ namespace Robust.Client.GameObjects
         [Dependency] private readonly IResourceCache _resourceCache = default!;
         [Dependency] private readonly ILogManager _logManager = default!;
         [Dependency] private readonly SharedTransformSystem _xforms = default!;
+        [Dependency] private readonly SpriteTreeSystem _tree = default!;
 
         public static readonly ProtoId<ShaderPrototype> UnshadedId = "unshaded";
         private readonly Queue<SpriteComponent> _inertUpdateQueue = new();
 
+        public static readonly ResPath TextureRoot = SpriteSpecifierSerializer.TextureRoot;
+
         /// <summary>
         ///     Entities that require a sprite frame update.
         /// </summary>
         private readonly HashSet<EntityUid> _queuedFrameUpdate = new();
 
         private ISawmill _sawmill = default!;
-
-        internal void Render(EntityUid uid, SpriteComponent sprite, DrawingHandleWorld drawingHandle, Angle eyeRotation, in Angle worldRotation, in Vector2 worldPosition)
-        {
-            if (!sprite.IsInert)
-                _queuedFrameUpdate.Add(uid);
-
-            sprite.RenderInternal(drawingHandle, eyeRotation, worldRotation, worldPosition, sprite.EnableDirectionOverride ? sprite.DirectionOverride : null);
-        }
+        private EntityQuery<SpriteComponent> _query;
 
         public override void Initialize()
         {
@@ -62,11 +57,11 @@ namespace Robust.Client.GameObjects
             UpdatesAfter.Add(typeof(SpriteTreeSystem));
 
             SubscribeLocalEvent<PrototypesReloadedEventArgs>(OnPrototypesReloaded);
-            SubscribeLocalEvent<SpriteComponent, SpriteUpdateInertEvent>(QueueUpdateInert);
             SubscribeLocalEvent<SpriteComponent, ComponentInit>(OnInit);
 
             Subs.CVar(_cfg, CVars.RenderSpriteDirectionBias, OnBiasChanged, true);
             _sawmill = _logManager.GetSawmill("sprite");
+            _query = GetEntityQuery<SpriteComponent>();
         }
 
         public bool IsVisible(Layer layer)
@@ -85,18 +80,6 @@ namespace Robust.Client.GameObjects
             SpriteComponent.DirectionBias = value;
         }
 
-        private void QueueUpdateInert(EntityUid uid, SpriteComponent sprite, ref SpriteUpdateInertEvent ev)
-            => QueueUpdateInert(uid, sprite);
-
-        public void QueueUpdateInert(EntityUid uid, SpriteComponent sprite)
-        {
-            if (sprite._inertUpdateQueued)
-                return;
-
-            sprite._inertUpdateQueued = true;
-            _inertUpdateQueue.Enqueue(sprite);
-        }
-
         private void DoUpdateIsInert(SpriteComponent component)
         {
             component._inertUpdateQueued = false;

Robust.Client/GameObjects/EntitySystems/VisualizerSystem.cs

@@ -11,6 +11,7 @@ public abstract class VisualizerSystem<T> : EntitySystem
 {
     [Dependency] protected readonly AppearanceSystem AppearanceSystem = default!;
     [Dependency] protected readonly AnimationPlayerSystem AnimationSystem = default!;
+    [Dependency] protected readonly SpriteSystem SpriteSystem = default!;
 
     public override void Initialize()
     {

Robust.Client/GameStates/ClientGameStateManager.cs

@@ -1306,6 +1306,11 @@ namespace Robust.Client.GameStates
                     meta.LastStateApplied = lastStateApplied.Value;
 
                 var xform = xforms.GetComponent(ent.Value);
+
+                // TODO PVS DETACH
+                // Why is this if block here again? If a null-space entity gets sent to a player via some PVS override,
+                // and then later on it gets removed, you would assume that the client marks it as detached?
+                // I.e., modifying the metadata flag & pausing the entity should probably happen outside of this block.
                 if (xform.ParentUid.IsValid())
                 {
                     lookupSys.RemoveFromEntityTree(ent.Value, xform);
@@ -1326,6 +1331,13 @@ namespace Robust.Client.GameStates
                     xformSys.DetachEntity(ent.Value, xform);
                     DebugTools.Assert((meta.Flags & MetaDataFlags.InContainer) == 0);
 
+                    // We mark the entity as paused, without raising a pause-event.
+                    // The entity gets un-paused when the metadata's comp-state is reapplied (which also does not raise
+                    // an un-pause event). The assumption is that game logic that has to handle the pausing should be
+                    // getting networked anyway. And if its some client-side timer on a networked entity, the timer
+                    // shouldn't actually be getting paused just because the entity has left the players view.
+                    meta.PauseTime = TimeSpan.Zero;
+
                     if (container != null)
                         containerSys.AddExpectedEntity(netEntity, container);
                 }

Robust.Client/Graphics/Clyde/Clyde.GridRendering.cs

@@ -412,8 +412,11 @@ namespace Robust.Client.Graphics.Clyde
         private void _updateTileMapOnUpdate(ref TileChangedEvent args)
         {
             var gridData = _mapChunkData.GetOrNew(args.Entity);
-            if (gridData.TryGetValue(args.ChunkIndex, out var data))
-                data.Dirty = true;
+            foreach (var change in args.Changes)
+            {
+                if (gridData.TryGetValue(change.ChunkIndex, out var data))
+                    data.Dirty = true;
+            }
         }
 
         private void _updateOnGridCreated(GridStartupEvent ev)

Robust.Client/Graphics/Clyde/Clyde.HLR.cs

@@ -318,7 +318,7 @@ namespace Robust.Client.Graphics.Clyde
                         screenSpriteSize.Y++;
 
                     bool exit = false;
-                    if (entry.Sprite.GetScreenTexture)
+                    if (entry.Sprite.GetScreenTexture && entry.Sprite.PostShader != null)
                     {
                         FlushRenderQueue();
                         var tex = CopyScreenTexture(viewport.RenderTarget);
@@ -369,7 +369,7 @@ namespace Robust.Client.Graphics.Clyde
                     }
                 }
 
-                spriteSystem.Render(entry.Uid, entry.Sprite, _renderHandle.DrawingHandleWorld, eye.Rotation, in entry.WorldRot, in entry.WorldPos);
+                spriteSystem.RenderSprite(new(entry.Uid, entry.Sprite), _renderHandle.DrawingHandleWorld, eye.Rotation, entry.WorldRot, entry.WorldPos);
 
                 if (entry.Sprite.PostShader != null && entityPostRenderTarget != null)
                 {

Robust.Client/Graphics/Clyde/Clyde.Rendering.cs

@@ -613,6 +613,8 @@ namespace Robust.Client.Graphics.Clyde
             EnsureBatchSpaceAvailable(4, GetQuadBatchIndexCount());
             EnsureBatchState(texture, true, GetQuadBatchPrimitiveType(), _queuedShader);
 
+            // TODO RENDERING
+            // It's probably better to do this on the GPU.
             bl = Vector2.Transform(bl, _currentMatrixModel);
             br = Vector2.Transform(br, _currentMatrixModel);
             tr = Vector2.Transform(tr, _currentMatrixModel);

Robust.Client/Graphics/Clyde/Clyde.Sprite.cs

@@ -153,7 +153,7 @@ internal partial class Clyde
 
             // special casing angle = n*pi/2 to avoid box rotation & bounding calculations doesn't seem to give significant speedups.
             data.SpriteScreenBB = TransformCenteredBox(
-                data.Sprite.Bounds,
+                _spriteSystem.GetLocalBounds((data.Uid, data.Sprite)),
                 finalRotation,
                 pos + batch.PreScaleViewOffset,
                 batch.ViewScale);

Robust.Client/Graphics/Clyde/Clyde.Systems.cs

@@ -10,6 +10,7 @@ internal sealed partial class Clyde
     private MapSystem _mapSystem = default!;
     private LightTreeSystem _lightTreeSystem = default!;
     private TransformSystem _transformSystem = default!;
+    private SpriteSystem _spriteSystem = default!;
     private SpriteTreeSystem _spriteTreeSystem = default!;
     private ClientOccluderSystem _occluderSystem = default!;
 
@@ -24,6 +25,7 @@ internal sealed partial class Clyde
         _mapSystem = _entitySystemManager.GetEntitySystem<MapSystem>();
         _lightTreeSystem = _entitySystemManager.GetEntitySystem<LightTreeSystem>();
         _transformSystem = _entitySystemManager.GetEntitySystem<TransformSystem>();
+        _spriteSystem = _entitySystemManager.GetEntitySystem<SpriteSystem>();
         _spriteTreeSystem = _entitySystemManager.GetEntitySystem<SpriteTreeSystem>();
         _occluderSystem = _entitySystemManager.GetEntitySystem<ClientOccluderSystem>();
     }
@@ -33,6 +35,7 @@ internal sealed partial class Clyde
         _mapSystem = null!;
         _lightTreeSystem = null!;
         _transformSystem = null!;
+        _spriteSystem = null!;
         _spriteTreeSystem = null!;
         _occluderSystem = null!;
     }

Robust.Client/Placement/PlacementManager.cs

@@ -47,6 +47,7 @@ namespace Robust.Client.Placement
 
         private SharedMapSystem Maps => EntityManager.System<SharedMapSystem>();
         private SharedTransformSystem XformSystem => EntityManager.System<SharedTransformSystem>();
+        private SpriteSystem Sprite => EntityManager.System<SpriteSystem>();
 
         /// <summary>
         ///     How long before a pending tile change is dropped.
@@ -359,12 +360,15 @@ namespace Robust.Client.Placement
 
         private void HandleTileChanged(ref TileChangedEvent args)
         {
-            var coords = Maps.GridTileToLocal(
-                args.NewTile.GridUid,
-                EntityManager.GetComponent<MapGridComponent>(args.NewTile.GridUid),
-                args.NewTile.GridIndices);
+            foreach (var change in args.Changes)
+            {
+                var coords = Maps.GridTileToLocal(
+                args.Entity,
+                args.Entity.Comp,
+                change.GridIndices);
 
-            _pendingTileChanges.RemoveAll(c => c.Item1 == coords);
+                _pendingTileChanges.RemoveAll(c => c.Item1 == coords);
+            }
         }
 
         /// <inheritdoc />
@@ -708,11 +712,11 @@ namespace Robust.Client.Placement
             CurrentPlacementOverlayEntity = null;
         }
 
-        private SpriteComponent SetupPlacementOverlayEntity()
+        private Entity<SpriteComponent> SetupPlacementOverlayEntity()
         {
             EnsureNoPlacementOverlayEntity();
             CurrentPlacementOverlayEntity = EntityManager.SpawnEntity(null, MapCoordinates.Nullspace);
-            return EntityManager.EnsureComponent<SpriteComponent>(CurrentPlacementOverlayEntity.Value);
+            return (CurrentPlacementOverlayEntity.Value, EntityManager.EnsureComponent<SpriteComponent>(CurrentPlacementOverlayEntity.Value));
         }
 
         private void PreparePlacement(string templateName)
@@ -729,10 +733,16 @@ namespace Robust.Client.Placement
                 EntityManager.GetComponent<MetaDataComponent>(CurrentPlacementOverlayEntity.Value));
         }
 
-        public void PreparePlacementSprite(SpriteComponent sprite)
+        public void PreparePlacementSprite(Entity<SpriteComponent> sprite)
         {
             var sc = SetupPlacementOverlayEntity();
-            sc.CopyFrom(sprite);
+            Sprite.CopySprite(sprite.AsNullable(), sc.AsNullable());
+        }
+
+        [Obsolete("Use the Entity<SpriteComponent> overload.")]
+        public void PreparePlacementSprite(SpriteComponent sprite)
+        {
+            PreparePlacementSprite((sprite.Owner, sprite));
         }
 
         public void PreparePlacementTexList(List<IDirectionalTextureProvider>? texs, bool noRot, EntityPrototype? prototype)
@@ -743,27 +753,27 @@ namespace Robust.Client.Placement
                 // This one covers most cases (including Construction)
                 foreach (var v in texs)
                 {
-                    if (v is RSI.State)
+                    if (v is RSI.State st)
                     {
-                        var st = (RSI.State) v;
-                        sc.AddLayer(st.StateId, st.RSI);
+                        Sprite.AddRsiLayer(sc.AsNullable(), st.StateId, st.RSI);
                     }
                     else
                     {
                         // Fallback
-                        sc.AddLayer(v.Default);
+                        Sprite.AddTextureLayer(sc.AsNullable(), v.Default);
                     }
                 }
             }
             else
             {
-                sc.AddLayer(new ResPath("/Textures/Interface/tilebuildoverlay.png"));
+                Sprite.AddTextureLayer(sc.AsNullable(), new ResPath("/Textures/Interface/tilebuildoverlay.png"));
             }
-            sc.NoRotation = noRot;
+
+            sc.Comp.NoRotation = noRot;
 
             if (prototype != null && prototype.TryGetComponent<SpriteComponent>("Sprite", out var spriteComp))
             {
-                sc.Scale = spriteComp.Scale;
+                Sprite.SetScale(sc.AsNullable(), spriteComp.Scale);
             }
 
         }
@@ -771,7 +781,7 @@ namespace Robust.Client.Placement
         private void PreparePlacementTile()
         {
             var sc = SetupPlacementOverlayEntity();
-            sc.AddLayer(new ResPath("/Textures/Interface/tilebuildoverlay.png"));
+            Sprite.AddTextureLayer(sc.AsNullable(), new ResPath("/Textures/Interface/tilebuildoverlay.png"));
 
             IsActive = true;
         }

Robust.Client/Placement/PlacementMode.cs

@@ -126,7 +126,7 @@ namespace Robust.Client.Placement
 
                 sprite.Color = IsValidPosition(coordinate) ? ValidPlaceColor : InvalidPlaceColor;
                 var rot = args.Viewport.Eye?.Rotation ?? default;
-                spriteSys.Render(uid.Value, sprite, args.WorldHandle, rot, worldRot, worldPos);
+                spriteSys.RenderSprite((uid.Value, sprite), args.WorldHandle, rot, worldRot, worldPos);
             }
         }
 

Robust.Client/Player/PlayerManager.cs

@@ -166,7 +166,7 @@ namespace Robust.Client.Player
             {
                 if (_client.RunLevel != ClientRunLevel.SinglePlayerGame)
                     Sawmill.Warning($"Attaching local player to an entity {EntManager.ToPrettyString(uid)} without an eye. This eye will not be netsynced and may cause issues.");
-                var eye = (EyeComponent) Factory.GetComponent(typeof(EyeComponent));
+                var eye = Factory.GetComponent<EyeComponent>();
                 eye.NetSyncEnabled = false;
                 EntManager.AddComponent(uid.Value, eye);
             }

Robust.Client/ResourceManagement/ResourceCache.Preload.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 using System.Threading.Tasks;
@@ -8,6 +9,7 @@ using Robust.Client.Graphics;
 using Robust.Client.Utility;
 using Robust.Shared;
 using Robust.Shared.Audio;
+using Robust.Shared.Collections;
 using Robust.Shared.Configuration;
 using Robust.Shared.ContentPack;
 using Robust.Shared.Graphics;
@@ -191,65 +193,143 @@ namespace Robust.Client.ResourceManagement
             // TODO allow RSIs to opt out (useful for very big & rare RSIs)
             // TODO combine with (non-rsi) texture atlas?
 
-            Array.Sort(atlasList, (b, a) => (b.AtlasSheet?.Height ?? 0).CompareTo(a.AtlasSheet?.Height ?? 0));
+            // We now need to insert the RSIs into the atlas. This specific problem is 2BP|O|F - the items are oriented
+            // and cutting is free. The sorting is done by a slightly modified FFDH algorithm. The algorithm is exactly
+            // the same as the standard FFDH algorithm with one main difference: We create new "levels" above placed
+            // blocks. For example if the first block was 10x20, then the second was 10x10 units, we would create a
+            // 10x10 level above the second block that would be treated as a normal level. This increases the packing
+            // efficiency from ~85% to ~95% with very little extra computational effort. The algorithm appears to be
+            // ~97% effective for storing SS14s RSIs.
+            //
+            // Here are some more resources about the strip packing problem!
+            //   -  https://en.wikipedia.org/w/index.php?title=Strip_packing_problem&oldid=1263496949#First-fit_decreasing-height_(FFDH)
+            //   -  https://www.csc.liv.ac.uk/~epa/surveyhtml.html
+            //   -  https://www.dei.unipd.it/~fisch/ricop/tesi/tesi_dottorato_Lodi_1999.pdf
+
+            // The array must be sorted from biggest to smallest first.
+            Array.Sort(atlasList, (b, a) => a.AtlasSheet.Height.CompareTo(b.AtlasSheet.Height));
 
-            // Each RSI sub atlas has a different size.
-            // Even if we iterate through them once to estimate total area, I have NFI how to sanely estimate an optimal square-texture size.
-            // So fuck it, just default to letting it be as large as it needs to and crop it as needed?
             var maxSize = Math.Min(GL.GetInteger(GetPName.MaxTextureSize), _configurationManager.GetCVar(CVars.ResRSIAtlasSize));
-            var sheet = new Image<Rgba32>(maxSize, maxSize);
 
-            var deltaY = 0;
-            Vector2i offset = default;
-            int finalized = -1;
-            int atlasCount = 0;
-            for (int i = 0; i < atlasList.Length; i++)
+            // THIS IS NOT GUARANTEED TO HAVE ANY PARTICULARLY LOGICAL ORDERING.
+            // E.G you could have atlas 1 RSIs appear *before* you're done seeing atlas 2 RSIs.
+            var levels = new ValueList<Level>();
+
+            // List of all the image atlases.
+            var imageAtlases = new ValueList<Image<Rgba32>>();
+
+            // List of all the actual atlases.
+            var finalAtlases = new ValueList<OwnedTexture>();
+
+            // Number of total pixels in each atlas.
+            var finalPixels = new ValueList<int>();
+
+            // First we just find the location of all the RSIs in the atlas before actually placing them.
+            // This allows us to effectively determine how much space we need to allocate for the images.
+            var currentHeight = 0;
+            var currentAtlasIndex = 0;
+            foreach (var rsi in atlasList)
             {
-                var rsi = atlasList[i];
-                if (rsi.Bad)
+                var insertHeight = rsi.AtlasSheet.Height;
+                var insertWidth = rsi.AtlasSheet.Width;
+
+                var found = false;
+                for (var i = 0; i < levels.Count && !found; i++)
+                {
+                    var levelPosition = levels[i].Position;
+                    var levelWidth = levels[i].Width;
+                    var levelHeight = levels[i].Height;
+
+                    // Check if it can fit in this level.
+                    if (levelHeight < insertHeight || levelWidth + insertWidth > levels[i].MaxWidth)
+                        continue;
+
+                    found = true;
+
+                    levels[i].Width += insertWidth;
+                    rsi.AtlasOffset = levelPosition + new Vector2i(levelWidth, 0);
+                    levels[i].RSIList.Add(rsi);
+
+                    // Creating the extra "free" space above blocks that can be used for inserting more items.
+                    // This differs from the FFDH spec which just ignores this space.
+                    Debug.Assert(levelHeight >= insertHeight); // Must be true because the array needs to be sorted
+                    if (levelHeight - insertHeight == 0)
+                        continue;
+
+                    var freeLevel = new Level
+                    {
+                        AtlasId = levels[i].AtlasId,
+                        Position = levelPosition + new Vector2i(levelWidth, insertHeight),
+                        Height = levelHeight - insertHeight,
+                        Width = 0,
+                        MaxWidth = insertWidth,
+                        RSIList = [ ]
+                    };
+
+                    levels.Add(freeLevel);
+                }
+
+                if (found)
                     continue;
 
-                DebugTools.Assert(rsi.AtlasSheet.Width < sheet.Width);
-                DebugTools.Assert(rsi.AtlasSheet.Height < sheet.Height);
-
-                if (offset.X + rsi.AtlasSheet.Width > sheet.Width)
+                // Ran out of space, we need to move on to the next atlas.
+                // This also isn't in the normal FFDH algorithm (obviously) but its close enough.
+                if (currentHeight + insertHeight > maxSize)
                 {
-                    offset.X = 0;
-                    offset.Y += deltaY;
+                    imageAtlases.Add(new Image<Rgba32>(maxSize, currentHeight));
+                    finalPixels.Add(0);
+                    currentHeight = 0;
+                    currentAtlasIndex++;
                 }
 
-                if (offset.Y + rsi.AtlasSheet.Height > sheet.Height)
-                {
-                    FinalizeMetaAtlas(i-1, sheet);
-                    sheet = new Image<Rgba32>(maxSize, maxSize);
-                    deltaY = 0;
-                    offset = default;
-                }
+                rsi.AtlasOffset = new Vector2i(0, currentHeight);
 
-                deltaY = Math.Max(deltaY, rsi.AtlasSheet.Height);
-                var box = new UIBox2i(0, 0, rsi.AtlasSheet.Width, rsi.AtlasSheet.Height);
-                rsi.AtlasSheet.Blit(box, sheet, offset);
-                rsi.AtlasOffset = offset;
-                offset.X += rsi.AtlasSheet.Width;
+                var newLevel = new Level
+                {
+                    AtlasId = currentAtlasIndex,
+                    Position = new Vector2i(0, currentHeight),
+                    Height = insertHeight,
+                    Width = insertWidth,
+                    MaxWidth = maxSize,
+                    RSIList = [ rsi ]
+                };
+                levels.Add(newLevel);
+
+                currentHeight += insertHeight;
             }
 
-            var height = offset.Y + deltaY;
-            var croppedSheet = new Image<Rgba32>(maxSize, height);
-            sheet.Blit(new UIBox2i(0, 0, maxSize, height), croppedSheet, default);
-            FinalizeMetaAtlas(atlasList.Length - 1, croppedSheet);
+            // This allocation takes a long time.
+            imageAtlases.Add(new Image<Rgba32>(maxSize, currentHeight));
+            finalPixels.Add(0);
 
-            void FinalizeMetaAtlas(int toIndex, Image<Rgba32> sheet)
+            // Put all textures on the atlases
+            foreach (var level in levels)
             {
-                var fromIndex = finalized + 1;
-                var atlas = Clyde.LoadTextureFromImage(sheet, $"Meta atlas {fromIndex}-{toIndex}");
-                for (int i = fromIndex; i <= toIndex; i++)
+                foreach (var rsi in level.RSIList)
                 {
-                    var rsi = atlasList[i];
-                    rsi.AtlasTexture = atlas;
-                }
+                    var box = new UIBox2i(0, 0, rsi.AtlasSheet.Width, rsi.AtlasSheet.Height);
 
-                finalized = toIndex;
-                atlasCount++;
+                    rsi.AtlasSheet.Blit(box, imageAtlases[level.AtlasId], rsi.AtlasOffset);
+                    finalPixels[level.AtlasId] += rsi.AtlasSheet.Width * rsi.AtlasSheet.Height;
+                }
+            }
+
+            // Finalize the atlases.
+            for (var i = 0; i < imageAtlases.Count; i++)
+            {
+                var atlasTexture = Clyde.LoadTextureFromImage(imageAtlases[i], $"Meta atlas {i}");
+                finalAtlases.Add(atlasTexture);
+
+                sawmill.Debug($"(Meta atlas {i}) - cropped utilization: {(float)finalPixels[i] / (maxSize * imageAtlases[i].Height):P2}, fill percentage: {(float)imageAtlases[i].Height / maxSize:P2}");
+            }
+
+            // Finally, reference the actual atlas from the RSIs.
+            foreach (var level in levels)
+            {
+                foreach (var rsi in level.RSIList)
+                {
+                    rsi.AtlasTexture = finalAtlases[level.AtlasId];
+                }
             }
 
             Parallel.ForEach(rsiList, data =>
@@ -294,7 +374,7 @@ namespace Robust.Client.ResourceManagement
             sawmill.Debug(
                 "Preloaded {CountLoaded} RSIs into {CountAtlas} Atlas(es?) ({CountNotAtlas} not atlassed, {CountErrored} errored) in {LoadTime}",
                 rsiList.Length,
-                atlasCount,
+                finalAtlases.Count,
                 nonAtlasList.Length,
                 errors,
                 sw.Elapsed);
@@ -305,4 +385,38 @@ namespace Robust.Client.ResourceManagement
             return rsi.MetaAtlas && rsi.LoadParameters == TextureLoadParameters.Default;
         }
     }
+
+    /// <summary>
+    ///     A "Level" to place boxes. Similar to FFDH levels, but with more parameters so we can fit in "free" levels
+    ///     above placed boxes.
+    /// </summary>
+    internal sealed class Level
+    {
+        /// <summary>
+        ///     Index of the atlas this is located.
+        /// </summary>
+        public required int AtlasId;
+        /// <summary>
+        ///     Bottom left of the location for the RSIs.
+        /// </summary>
+        public required Vector2i Position;
+        /// <summary>
+        ///     The current width of the level.
+        /// </summary>
+        /// <remarks>This can (and will) be 0. Will change.</remarks>
+        public required int Width;
+        /// <summary>
+        ///     The current height of the level.
+        /// </summary>
+        /// <remarks>This value should never change.</remarks>
+        public required int Height;
+        /// <summary>
+        ///     Maximum width of the level.
+        /// </summary>
+        public required int MaxWidth;
+        /// <summary>
+        ///     List of all the RSIs stored in this level. RSIs are ordered from tallest to smallest per level.
+        /// </summary>
+        public required List<RSIResource.LoadStepData> RSIList;
+    }
 }

Robust.Client/UserInterface/Controls/OutputPanel.cs

@@ -95,6 +95,12 @@ namespace Robust.Client.UserInterface.Controls
         public void Clear()
         {
             _firstLine = true;
+
+            foreach (var entry in _entries)
+            {
+                entry.RemoveControls();
+            }
+
             _entries.Clear();
             _totalContentHeight = 0;
             _scrollBar.MaxValue = Math.Max(_scrollBar.Page, _totalContentHeight);
@@ -104,6 +110,7 @@ namespace Robust.Client.UserInterface.Controls
         public void RemoveEntry(Index index)
         {
             var entry = _entries[index];
+            entry.RemoveControls();
             _entries.RemoveAt(index.GetOffset(_entries.Count));
 
             var font = _getFont();
@@ -189,6 +196,9 @@ namespace Robust.Client.UserInterface.Controls
                 if (entryOffset > contentBox.Height)
                 {
                     entry.HideControls();
+
+                    // We know that every subsequent entry will also fail the test, but we also need to
+                    // hide all the controls, so we cannot simply break out of the loop
                     continue;
                 }
 

Robust.Client/UserInterface/Controls/ProgressBar.cs

@@ -14,6 +14,27 @@ namespace Robust.Client.UserInterface.Controls
         private StyleBox? _backgroundStyleBoxOverride;
         private StyleBox? _foregroundStyleBoxOverride;
 
+        private bool _vertical;
+
+        /// <summary>
+        /// Whether the progress bar is oriented vertically.
+        /// </summary>
+        /// <remarks>
+        /// A vertical progress bar fills from bottom to top.
+        /// </remarks>
+        public bool Vertical
+        {
+            get => _vertical;
+            set
+            {
+                if (_vertical != value)
+                {
+                    _vertical = value;
+                    InvalidateMeasure();
+                }
+            }
+        }
+
         public StyleBox? BackgroundStyleBoxOverride
         {
             get => _backgroundStyleBoxOverride;
@@ -70,11 +91,23 @@ namespace Robust.Client.UserInterface.Controls
             {
                 return;
             }
-            var minSize = fg.MinimumSize;
-            var size = PixelWidth * GetAsRatio() - minSize.X;
-            if (size > 0)
+
+            if (_vertical)
             {
-                fg.Draw(handle, UIBox2.FromDimensions(0, 0, minSize.X + size, PixelHeight), UIScale);
+                var size = PixelHeight * GetAsRatio();
+                if (size > 0)
+                {
+                    fg.Draw(handle, UIBox2.FromDimensions(0, PixelHeight - size, PixelWidth, size), UIScale);
+                }
+            }
+            else
+            {
+                var minSize = fg.MinimumSize;
+                var size = PixelWidth * GetAsRatio() - minSize.X;
+                if (size > 0)
+                {
+                    fg.Draw(handle, UIBox2.FromDimensions(0, 0, minSize.X + size, PixelHeight), UIScale);
+                }
             }
         }
 

Robust.Client/UserInterface/Controls/RichTextLabel.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Numerics;
 using JetBrains.Annotations;
 using Robust.Client.Graphics;
@@ -15,8 +17,7 @@ namespace Robust.Client.UserInterface.Controls
     {
         [Dependency] private readonly MarkupTagManager _tagManager = default!;
 
-        private FormattedMessage? _message;
-        private RichTextEntry _entry;
+        private RichTextEntry? _entry;
         private float _lineHeightScale = 1;
         private bool _lineHeightOverride;
 
@@ -40,19 +41,26 @@ namespace Robust.Client.UserInterface.Controls
 
         public string? Text
         {
-            get => _message?.ToMarkup();
+            get => _entry?.Message.ToMarkup();
             set
             {
                 if (value == null)
-                {
-                    _message?.Clear();
-                    return;
-                }
-
-                SetMessage(FormattedMessage.FromMarkupPermissive(value));
+                    Clear();
+                else
+                    SetMessage(FormattedMessage.FromMarkupPermissive(value));
             }
         }
 
+        public void Clear()
+        {
+            _entry?.RemoveControls();
+            _entry = null;
+            InvalidateMeasure();
+        }
+
+        public IEnumerable<Control> Controls => _entry?.Controls?.Values ?? Enumerable.Empty<Control>();
+        public IReadOnlyList<MarkupNode> Nodes => _entry?.Message.Nodes ?? Array.Empty<MarkupNode>();
+
         public RichTextLabel()
         {
             IoCManager.InjectDependencies(this);
@@ -61,8 +69,8 @@ namespace Robust.Client.UserInterface.Controls
 
         public void SetMessage(FormattedMessage message, Type[]? tagsAllowed = null, Color? defaultColor = null)
         {
-            _message = message;
-            _entry = new RichTextEntry(_message, this, _tagManager, tagsAllowed, defaultColor);
+            _entry?.RemoveControls();
+            _entry = new RichTextEntry(message, this, _tagManager, tagsAllowed, defaultColor);
             InvalidateMeasure();
         }
 
@@ -73,31 +81,31 @@ namespace Robust.Client.UserInterface.Controls
             SetMessage(msg, tagsAllowed, defaultColor);
         }
 
-        public string? GetMessage() => _message?.ToMarkup();
+        public string? GetMessage() => _entry?.Message.ToMarkup();
+
+        /// <summary>
+        /// Returns a copy of the currently used formatted message.
+        /// </summary>
+        public FormattedMessage? GetFormattedMessage() => _entry == null ? null : new FormattedMessage(_entry.Value.Message);
 
         protected override Vector2 MeasureOverride(Vector2 availableSize)
         {
-            if (_message == null)
-            {
+            if (_entry == null)
                 return Vector2.Zero;
-            }
 
             var font = _getFont();
-            _entry.Update(_tagManager, font, availableSize.X * UIScale, UIScale, LineHeightScale);
 
-            return new Vector2(_entry.Width / UIScale, _entry.Height / UIScale);
+            // _entry is nullable struct.
+            // cannot just call _entry.Value.Update() as that doesn't actually update _entry.
+            _entry = _entry.Value.Update(_tagManager, font, availableSize.X * UIScale, UIScale, LineHeightScale);
+
+            return new Vector2(_entry.Value.Width / UIScale, _entry.Value.Height / UIScale);
         }
 
         protected internal override void Draw(DrawingHandleScreen handle)
         {
             base.Draw(handle);
-
-            if (_message == null)
-            {
-                return;
-            }
-
-            _entry.Draw(_tagManager, handle, _getFont(), SizeBox, 0, new MarkupDrawingContext(), UIScale, LineHeightScale);
+            _entry?.Draw(_tagManager, handle, _getFont(), SizeBox, 0, new MarkupDrawingContext(), UIScale, LineHeightScale);
         }
 
         [Pure]

Robust.Client/UserInterface/RichText/BoldItalicTag.cs

@@ -5,7 +5,7 @@ using Robust.Shared.Utility;
 
 namespace Robust.Client.UserInterface.RichText;
 
-public sealed class BoldItalicTag : IMarkupTag
+public sealed class BoldItalicTag : IMarkupTagHandler
 {
     public const string BoldItalicFont = "DefaultBoldItalic";
 

Robust.Client/UserInterface/RichText/BoldTag.cs

@@ -6,7 +6,7 @@ using Robust.Shared.Utility;
 
 namespace Robust.Client.UserInterface.RichText;
 
-public sealed class BoldTag : IMarkupTag
+public sealed class BoldTag : IMarkupTagHandler
 {
     public const string BoldFont = "DefaultBold";
 

Robust.Client/UserInterface/RichText/BulletTag.cs

@@ -2,7 +2,7 @@ using Robust.Shared.Utility;
 
 namespace Robust.Client.UserInterface.RichText;
 
-public sealed class BulletTag : IMarkupTag
+public sealed class BulletTag : IMarkupTagHandler
 {
     public string Name => "bullet";
 

Robust.Client/UserInterface/RichText/ColorTag.cs

@@ -6,7 +6,7 @@ namespace Robust.Client.UserInterface.RichText;
 /// <summary>
 /// Colors the text inside its opening and closing nodes
 /// </summary>
-public sealed class ColorTag : IMarkupTag
+public sealed class ColorTag : IMarkupTagHandler
 {
     public static readonly Color DefaultColor = new(200, 200, 200);
 

Robust.Client/UserInterface/RichText/CommandLinkTag.cs

@@ -8,14 +8,14 @@ using Robust.Shared.Utility;
 
 namespace Robust.Client.UserInterface.RichText;
 
-public sealed class CommandLinkTag : IMarkupTag
+public sealed class CommandLinkTag : IMarkupTagHandler
 {
     [Dependency] private readonly IClientConsoleHost _clientConsoleHost = default!;
 
     public string Name => "cmdlink";
 
     /// <inheritdoc/>
-    public bool TryGetControl(MarkupNode node, [NotNullWhen(true)] out Control? control)
+    public bool TryCreateControl(MarkupNode node, [NotNullWhen(true)] out Control? control)
     {
         if (!node.Value.TryGetString(out var text)
             || !node.Attributes.TryGetValue("command", out var commandParameter)

Robust.Client/UserInterface/RichText/FontTag.cs

@@ -11,7 +11,7 @@ namespace Robust.Client.UserInterface.RichText;
 /// Applies the font provided as the tags parameter to the markup drawing context.
 /// Definitely not save for user supplied markup
 /// </summary>
-public sealed class FontTag : IMarkupTag
+public sealed class FontTag : IMarkupTagHandler
 {
     public const string DefaultFont = "Default";
     public const int DefaultSize = 12;

Robust.Client/UserInterface/RichText/HeadingTag.cs

@@ -6,7 +6,7 @@ using Robust.Shared.Utility;
 
 namespace Robust.Client.UserInterface.RichText;
 
-public sealed class HeadingTag : IMarkupTag
+public sealed class HeadingTag : IMarkupTagHandler
 {
     [Dependency] private readonly IResourceCache _resourceCache = default!;
     [Dependency] private readonly IPrototypeManager _prototypeManager = default!;

Robust.Client/UserInterface/RichText/IMarkupTag.cs

@@ -1,9 +1,16 @@
+using System;
 using System.Diagnostics.CodeAnalysis;
 using Robust.Shared.Utility;
 
 namespace Robust.Client.UserInterface.RichText;
 
-public interface IMarkupTag
+/// <summary>
+/// Classes that implement this interface will be instantiated by <see cref="MarkupTagManager"/> and used to handle
+/// the parsing and behaviour of markup tags. Note that each class is only ever instantiated once by the tag manager,
+/// and wil be used to handle all tags of that kind, and thus should not contain state information relevant to a
+/// specific tag.
+/// </summary>
+public interface IMarkupTagHandler
 {
     /// <summary>
     /// The string used as the tags name when writing rich text
@@ -54,17 +61,32 @@ public interface IMarkupTag
     }
 
     /// <summary>
-    /// Called inside the constructor of <see cref="RichTextEntry"/> to
-    /// supply a control that gets rendered inline before this tags children<br/>
-    /// Text continues to the right of the control until the next line and then continues bellow it
+    /// Called inside the constructor of <see cref="RichTextEntry"/> to supply a control that gets rendered inline
+    /// before this tags children. The returned control must be new instance to avoid issues with shallow cloning
+    /// <see cref="FormattedMessage"/> nodes. Text continues to the right of the control until the next line and
+    /// then continues bellow it.
     /// </summary>
     /// <param name="node">The markup node containing the parameter and attributes</param>
     /// <param name="control">A UI control for placing in line with this tags children</param>
     /// <returns>true if this tag supplies a control</returns>
+    public bool TryCreateControl(MarkupNode node, [NotNullWhen(true)] out Control? control)
+    {
+        control = null;
+        return false;
+    }
+}
+
+[Obsolete("Use IMarkupTagHandler")]
+public interface IMarkupTag : IMarkupTagHandler
+{
+    bool IMarkupTagHandler.TryCreateControl(MarkupNode node, [NotNullWhen(true)] out Control? control)
+    {
+        return TryGetControl(node, out control);
+    }
+
     public bool TryGetControl(MarkupNode node, [NotNullWhen(true)] out Control? control)
     {
         control = null;
         return false;
     }
-
 }

Robust.Client/UserInterface/RichText/ItalicTag.cs

@@ -6,7 +6,7 @@ using Robust.Shared.Utility;
 
 namespace Robust.Client.UserInterface.RichText;
 
-public sealed class ItalicTag : IMarkupTag
+public sealed class ItalicTag : IMarkupTagHandler
 {
     public const string ItalicFont = "DefaultItalic";
 

Robust.Client/UserInterface/RichText/MarkupTagManager.cs

@@ -16,7 +16,7 @@ public sealed class MarkupTagManager
     /// <summary>
     /// Tags defined in engine need to be instantiated here because of sandboxing
     /// </summary>
-    private readonly Dictionary<string, IMarkupTag> _markupTagTypes = new IMarkupTag[] {
+    private readonly Dictionary<string, IMarkupTagHandler> _markupTagTypes = new IMarkupTagHandler[] {
         new BoldItalicTag(),
         new BoldTag(),
         new BulletTag(),
@@ -44,13 +44,13 @@ public sealed class MarkupTagManager
 
     public void Initialize()
     {
-        foreach (var type in _reflectionManager.GetAllChildren<IMarkupTag>())
+        foreach (var type in _reflectionManager.GetAllChildren<IMarkupTagHandler>())
         {
             //Prevent tags defined inside engine from being instantiated
             if (_engineTypes.Contains(type))
                 continue;
 
-            var instance = (IMarkupTag)_sandboxHelper.CreateInstance(type);
+            var instance = (IMarkupTagHandler)_sandboxHelper.CreateInstance(type);
             _markupTagTypes[instance.Name.ToLower()] = instance;
         }
 
@@ -60,22 +60,48 @@ public sealed class MarkupTagManager
         }
     }
 
+    [Obsolete("Use GetMarkupTagHandler")]
     public IMarkupTag? GetMarkupTag(string name)
+    {
+        return _markupTagTypes.GetValueOrDefault(name) as IMarkupTag;
+    }
+
+    public IMarkupTagHandler? GetMarkupTagHandler(string name)
     {
         return _markupTagTypes.GetValueOrDefault(name);
     }
 
-    public bool TryGetMarkupTag(string name, Type[]? tagsAllowed, [NotNullWhen(true)] out IMarkupTag? tag)
+    /// <summary>
+    /// Attempt to get the tag handler with the corresponding name.
+    /// </summary>
+    /// <param name="name">The name of the tag, as specified by <see cref="IMarkupTag.Name"/></param>
+    /// <param name="tagsAllowed">List of allowed tag types. If null, all types are allowed.</param>
+    /// <param name="handler">The instance responsible for handling tags of this type.</param>
+    /// <returns></returns>
+    public bool TryGetMarkupTagHandler(string name, Type[]? tagsAllowed, [NotNullWhen(true)] out IMarkupTagHandler? handler)
     {
         if (_markupTagTypes.TryGetValue(name, out var markupTag)
             // Using a whitelist prevents new tags from sneaking in.
             && (tagsAllowed == null || Array.IndexOf(tagsAllowed, markupTag.GetType()) != -1))
         {
-            tag = markupTag;
+            handler = markupTag;
             return true;
         }
 
-        tag = null;
+        handler = null;
         return false;
     }
+
+    [Obsolete("Use TryGetMarkupTagHandler")]
+    public bool TryGetMarkupTag(string name, Type[]? tagsAllowed, [NotNullWhen(true)] out IMarkupTag? tag)
+    {
+        if (!TryGetMarkupTagHandler(name, tagsAllowed, out var handler) || handler is not IMarkupTag cast)
+        {
+            tag = null;
+            return false;
+        }
+
+        tag = cast;
+        return true;
+    }
 }

Robust.Client/UserInterface/RichTextEntry.cs

@@ -13,6 +13,9 @@ namespace Robust.Client.UserInterface
 {
     /// <summary>
     ///     Used by <see cref="OutputPanel"/> and <see cref="RichTextLabel"/> to handle rich text layout.
+    ///     Note that if this text is ever removed or modified without removing the owning control,
+    ///     then <see cref="RemoveControls"/> should be called to ensure that any controls that were added by this
+    ///     entry are also removed.
     /// </summary>
     internal struct RichTextEntry
     {
@@ -36,7 +39,7 @@ namespace Robust.Client.UserInterface
         /// </summary>
         public ValueList<int> LineBreaks;
 
-        private readonly Dictionary<int, Control>? _tagControls;
+        public readonly Dictionary<int, Control>? Controls;
 
         public RichTextEntry(FormattedMessage message, Control parent, MarkupTagManager tagManager, Type[]? tagsAllowed = null, Color? defaultColor = null)
         {
@@ -56,15 +59,35 @@ namespace Robust.Client.UserInterface
                 if (node.Name == null)
                     continue;
 
-                if (!tagManager.TryGetMarkupTag(node.Name, _tagsAllowed, out var tag) || !tag.TryGetControl(node, out var control))
+                if (!tagManager.TryGetMarkupTagHandler(node.Name, _tagsAllowed, out var handler) || !handler.TryCreateControl(node, out var control))
                     continue;
 
+                // Markup tag handler instances are shared across controls. We need to ensure that the hanlder doesn't
+                // store state information and return the same control for each rich text entry.
+                DebugTools.Assert(handler.TryCreateControl(node, out var other) && other != control);
+
                 parent.Children.Add(control);
                 tagControls ??= new Dictionary<int, Control>();
                 tagControls.Add(nodeIndex, control);
             }
 
-            _tagControls = tagControls;
+            Controls = tagControls;
+        }
+
+        // TODO RICH TEXT
+        // Somehow ensure that this **has** to be called when removing rich text from some control.
+        /// <summary>
+        /// Remove all owned controls from their parents.
+        /// </summary>
+        public readonly void RemoveControls()
+        {
+            if (Controls == null)
+                return;
+
+            foreach (var ctrl in Controls.Values)
+            {
+                ctrl.Orphan();
+            }
         }
 
         /// <summary>
@@ -74,7 +97,7 @@ namespace Robust.Client.UserInterface
         /// <param name="maxSizeX">The maximum horizontal size of the container of this entry.</param>
         /// <param name="uiScale"></param>
         /// <param name="lineHeightScale"></param>
-        public void Update(MarkupTagManager tagManager, Font defaultFont, float maxSizeX, float uiScale, float lineHeightScale = 1)
+        public RichTextEntry Update(MarkupTagManager tagManager, Font defaultFont, float maxSizeX, float uiScale, float lineHeightScale = 1)
         {
             // This method is gonna suck due to complexity.
             // Bear with me here.
@@ -112,10 +135,10 @@ namespace Robust.Client.UserInterface
                         continue;
 
                     if (ProcessMetric(ref this, metrics, out breakLine))
-                        return;
+                        return this;
                 }
 
-                if (_tagControls == null || !_tagControls.TryGetValue(nodeIndex, out var control))
+                if (Controls == null || !Controls.TryGetValue(nodeIndex, out var control))
                     continue;
 
                 control.Measure(new Vector2(Width, Height));
@@ -128,12 +151,14 @@ namespace Robust.Client.UserInterface
                     desiredSize.Y);
 
                 if (ProcessMetric(ref this, controlMetrics, out breakLine))
-                    return;
+                    return this;
             }
 
             Width = wordWrap.FinalizeText(out breakLine);
             CheckLineBreak(ref this, breakLine);
 
+            return this;
+
             bool ProcessRune(ref RichTextEntry src, Rune rune, out int? outBreakLine)
             {
                 wordWrap.NextRune(rune, out breakLine, out var breakNewLine, out var skip);
@@ -166,9 +191,10 @@ namespace Robust.Client.UserInterface
 
         internal readonly void HideControls()
         {
-            if (_tagControls == null)
+            if (Controls == null)
                 return;
-            foreach (var control in _tagControls.Values)
+
+            foreach (var control in Controls.Values)
             {
                 control.Visible = false;
             }
@@ -220,7 +246,7 @@ namespace Robust.Client.UserInterface
                     globalBreakCounter += 1;
                 }
 
-                if (_tagControls == null || !_tagControls.TryGetValue(nodeIndex, out var control))
+                if (Controls == null || !Controls.TryGetValue(nodeIndex, out var control))
                     continue;
 
                 // Controls may have been previously hidden via HideControls due to being "out-of frame".
@@ -243,7 +269,7 @@ namespace Robust.Client.UserInterface
                 return node.Value.StringValue ?? "";
 
             //Skip the node if there is no markup tag for it.
-            if (!tagManager.TryGetMarkupTag(node.Name, _tagsAllowed, out var tag))
+            if (!tagManager.TryGetMarkupTagHandler(node.Name, _tagsAllowed, out var tag))
                 return "";
 
             if (!node.Closing)

Robust.Client/Utility/SpriteSpecifierExt.cs

@@ -17,6 +17,7 @@ namespace Robust.Client.Utility
     /// </summary>
     public static class SpriteSpecifierExt
     {
+        [Obsolete("Use SpriteSystem.GetTexture() instead")]
         public static Texture GetTexture(this SpriteSpecifier.Texture texSpecifier, IResourceCache cache)
         {
             return cache
@@ -24,13 +25,14 @@ namespace Robust.Client.Utility
                 .Texture;
         }
 
-        [Obsolete("Use SpriteSystem")]
+        [Obsolete("Use SpriteSystem.GetState() instead")]
         public static RSI.State GetState(this SpriteSpecifier.Rsi rsiSpecifier, IResourceCache cache)
         {
             if (!cache.TryGetResource<RSIResource>(SpriteSpecifierSerializer.TextureRoot / rsiSpecifier.RsiPath, out var theRsi))
             {
+                var sys = IoCManager.Resolve<IEntitySystemManager>().GetEntitySystem<SpriteSystem>();
                 Logger.Error("SpriteSpecifier failed to load RSI {0}", rsiSpecifier.RsiPath);
-                return SpriteComponent.GetFallbackState(cache);
+                return sys.GetFallbackState();
             }
 
             if (theRsi.RSI.TryGetState(rsiSpecifier.RsiState, out var state))
@@ -39,21 +41,22 @@ namespace Robust.Client.Utility
             }
 
             Logger.Error($"SpriteSpecifier has invalid RSI state '{rsiSpecifier.RsiState}' for RSI: {rsiSpecifier.RsiPath}");
-            return SpriteComponent.GetFallbackState(cache);
+            return IoCManager.Resolve<IEntitySystemManager>().GetEntitySystem<SpriteSystem>().GetFallbackState();
         }
 
-        [Obsolete("Use SpriteSystem")]
+        [Obsolete("Use SpriteSystem.Frame0() instead")]
         public static Texture Frame0(this SpriteSpecifier specifier)
         {
             return specifier.RsiStateLike().Default;
         }
 
+        [Obsolete("Use SpriteSystem.RsiStateLike() instead")]
         public static IDirectionalTextureProvider DirFrame0(this SpriteSpecifier specifier)
         {
             return specifier.RsiStateLike();
         }
 
-        [Obsolete("Use SpriteSystem")]
+        [Obsolete("Use SpriteSystem.RsiStateLike() instead")]
         public static IRsiStateLike RsiStateLike(this SpriteSpecifier specifier)
         {
             var resC = IoCManager.Resolve<IResourceCache>();
@@ -67,10 +70,11 @@ namespace Robust.Client.Utility
 
                 case SpriteSpecifier.EntityPrototype prototypeIcon:
                     var protMgr = IoCManager.Resolve<IPrototypeManager>();
+                    var sys = IoCManager.Resolve<IEntitySystemManager>().GetEntitySystem<SpriteSystem>();
                     if (!protMgr.TryIndex<EntityPrototype>(prototypeIcon.EntityPrototypeId, out var prototype))
                     {
                         Logger.Error("Failed to load PrototypeIcon {0}", prototypeIcon.EntityPrototypeId);
-                        return SpriteComponent.GetFallbackState(resC);
+                        return sys.GetFallbackState();
                     }
 
                     return SpriteComponent.GetPrototypeIcon(prototype, resC);

Robust.Roslyn.Shared/Diagnostics.cs

@@ -39,6 +39,7 @@ public static class Diagnostics
     public const string IdForbidLiteral = "RA0033";
     public const string IdObsoleteInheritance = "RA0034";
     public const string IdObsoleteInheritanceWithMessage = "RA0035";
+    public const string IdDataFieldYamlSerializable = "RA0036";
 
     public static SuppressionDescriptor MeansImplicitAssignment =>
         new SuppressionDescriptor("RADC1000", "CS0649", "Marked as implicitly assigned.");

Robust.Server/BaseServer.cs

@@ -297,7 +297,7 @@ namespace Robust.Server
                 : null;
 
             // Set up the VFS
-            _resources.Initialize(dataDir);
+            _resources.Initialize(dataDir, hideUserDataDir: false);
 
             var mountOptions = _commandLineArgs != null
                 ? MountOptions.Merge(_commandLineArgs.MountOptions, Options.MountOptions) : Options.MountOptions;

Robust.Shared.Maths/Angle.cs

@@ -104,6 +104,16 @@ namespace Robust.Shared.Maths
             return (Direction) (Math.Floor((ang + CardinalOffset) / CardinalSegment) * 2 % 8);
         }
 
+        /// <summary>
+        /// Rounds the angle to the nearest cardinal direction. This behaves similarly to a combination of
+        /// <see cref="GetCardinalDir"/> and Direction.ToAngle(), however this may return an angle outside of the range
+        /// returned by those methods (-pi to pi).
+        /// </summary>
+        public Angle RoundToCardinalAngle()
+        {
+            return new Angle(CardinalSegment * Math.Floor((Theta + CardinalOffset) / CardinalSegment));
+        }
+
         /// <summary>
         ///     Rotates the vector counter-clockwise around its origin by the value of Theta.
         /// </summary>

Robust.Shared.Maths/Properties/AssemblyInfo.cs

@@ -6,4 +6,3 @@
 
 [assembly: InternalsVisibleTo("Robust.Client")]
 [assembly: InternalsVisibleTo("Robust.UnitTesting")]
-[assembly: InternalsVisibleTo("Content.Benchmarks")]

Robust.Shared/ComponentTrees/ComponentTreeSystem.cs

@@ -111,6 +111,11 @@ public abstract class ComponentTreeSystem<TTreeComp, TComp> : EntitySystem
         component.TreeUpdateQueued = true;
         _updateQueue.Enqueue((component, xform));
     }
+
+    public void QueueTreeUpdate(Entity<TComp> entity, TransformComponent? xform = null)
+    {
+        QueueTreeUpdate(entity.Owner, entity.Comp, xform);
+    }
     #endregion
 
     #region Component Management

Robust.Shared/ContentPack/AssemblyTypeChecker.Config.cs

@@ -88,6 +88,7 @@ namespace Robust.Shared.ContentPack
             public string SystemAssemblyName = default!;
             public HashSet<VerifierError> AllowedVerifierErrors = default!;
             public List<string> WhitelistedNamespaces = default!;
+            public List<string> AllowedAssemblyPrefixes = default!;
             public Dictionary<string, Dictionary<string, TypeConfig>> Types = default!;
         }
 

Robust.Shared/ContentPack/AssemblyTypeChecker.cs

@@ -131,6 +131,16 @@ namespace Robust.Shared.ContentPack
                 return false;
             }
 
+#pragma warning disable RA0004
+            var loadedConfig = _config.Result;
+#pragma warning restore RA0004
+
+            if (!loadedConfig.AllowedAssemblyPrefixes.Any(allowedNamePrefix => asmName.StartsWith(allowedNamePrefix)))
+            {
+                _sawmill.Error($"Assembly name '{asmName}' is not allowed for a content assembly");
+                return false;
+            }
+
             if (VerifyIL)
             {
                 if (!DoVerifyIL(asmName, resolver, peReader, reader))
@@ -179,10 +189,6 @@ namespace Robust.Shared.ContentPack
                 return true;
             }
 
-#pragma warning disable RA0004
-            var loadedConfig = _config.Result;
-#pragma warning restore RA0004
-
             var badRefs = new ConcurrentBag<EntityHandle>();
 
             // We still do explicit type reference scanning, even though the actual whitelists work with raw members.

Robust.Shared/ContentPack/DirLoader.cs

@@ -60,9 +60,7 @@ namespace Robust.Shared.ContentPack
 
             internal string GetPath(ResPath relPath)
             {
-                return Path.GetFullPath(Path.Combine(_directory.FullName, relPath.ToRelativeSystemPath()))
-                    // Sanitise platform-specific path and standardize it for engine use.
-                    .Replace(Path.DirectorySeparatorChar, '/');
+                return PathHelpers.SafeGetResourcePath(_directory.FullName, relPath);
             }
 
             /// <inheritdoc />

Robust.Shared/ContentPack/IResourceManagerInternal.cs

@@ -14,7 +14,11 @@ namespace Robust.Shared.ContentPack
         /// The directory to use for user data.
         /// If null, a virtual temporary file system is used instead.
         /// </param>
-        void Initialize(string? userData);
+        /// <param name="hideUserDataDir">
+        /// If true, <see cref="IWritableDirProvider.RootDir"/> will be hidden on
+        /// <see cref="IResourceManager.UserData"/>.
+        /// </param>
+        void Initialize(string? userData, bool hideUserDataDir);
 
         /// <summary>
         ///     Mounts a single stream as a content file. Useful for unit testing.

Robust.Shared/ContentPack/IWritableDirProvider.cs

@@ -13,7 +13,7 @@ namespace Robust.Shared.ContentPack
     {
         /// <summary>
         /// The root path of this provider.
-        /// Can be null if it's a virtual provider.
+        /// Can be null if it's a virtual provider or the path is protected (e.g. on the client).
         /// </summary>
         string? RootDir { get; }
 

Robust.Shared/ContentPack/ModLoader.cs

@@ -93,19 +93,23 @@ namespace Robust.Shared.ContentPack
         {
             var sw = Stopwatch.StartNew();
             Sawmill.Debug("LOADING modules");
-            var files = new Dictionary<string, (ResPath Path, string[] references)>();
+            var files = new Dictionary<string, (ResPath Path, MemoryStream data, string[] references)>();
 
             // Find all modules we want to load.
             foreach (var fullPath in paths)
             {
                 using var asmFile = _res.ContentFileRead(fullPath);
-                var refData = GetAssemblyReferenceData(asmFile);
+                var ms = new MemoryStream();
+                asmFile.CopyTo(ms);
+
+                ms.Position = 0;
+                var refData = GetAssemblyReferenceData(ms);
                 if (refData == null)
                     continue;
 
                 var (asmRefs, asmName) = refData.Value;
 
-                if (!files.TryAdd(asmName, (fullPath, asmRefs)))
+                if (!files.TryAdd(asmName, (fullPath, ms, asmRefs)))
                 {
                     Sawmill.Error("Found multiple modules with the same assembly name " +
                                   $"'{asmName}', A: {files[asmName].Path}, B: {fullPath}.");
@@ -122,10 +126,10 @@ namespace Robust.Shared.ContentPack
 
                 Parallel.ForEach(files, pair =>
                 {
-                    var (name, (path, _)) = pair;
+                    var (name, (_, data, _)) = pair;
 
-                    using var stream = _res.ContentFileRead(path);
-                    if (!typeChecker.CheckAssembly(stream, resolver))
+                    data.Position = 0;
+                    if (!typeChecker.CheckAssembly(data, resolver))
                     {
                         throw new TypeCheckFailedException($"Assembly {name} failed type checks.");
                     }
@@ -137,14 +141,15 @@ namespace Robust.Shared.ContentPack
             var nodes = TopologicalSort.FromBeforeAfter(
                 files,
                 kv => kv.Key,
-                kv => kv.Value.Path,
+                kv => kv.Value,
                 _ => Array.Empty<string>(),
                 kv => kv.Value.references,
                 allowMissing: true); // missing refs would be non-content assemblies so allow that.
 
             // Actually load them in the order they depend on each other.
-            foreach (var path in TopologicalSort.Sort(nodes))
+            foreach (var item in TopologicalSort.Sort(nodes))
             {
+                var (path, memory, _) = item;
                 Sawmill.Debug($"Loading module: '{path}'");
                 try
                 {
@@ -156,9 +161,9 @@ namespace Robust.Shared.ContentPack
                     }
                     else
                     {
-                        using var assemblyStream = _res.ContentFileRead(path);
+                        memory.Position = 0;
                         using var symbolsStream = _res.ContentFileReadOrNull(path.WithExtension("pdb"));
-                        LoadGameAssembly(assemblyStream, symbolsStream, skipVerify: true);
+                        LoadGameAssembly(memory, symbolsStream, skipVerify: true);
                     }
                 }
                 catch (Exception e)
@@ -174,7 +179,7 @@ namespace Robust.Shared.ContentPack
 
         private (string[] refs, string name)? GetAssemblyReferenceData(Stream stream)
         {
-            using var reader = ModLoader.MakePEReader(stream);
+            using var reader = ModLoader.MakePEReader(stream, leaveOpen: true);
             var metaReader = reader.GetMetadataReader();
 
             var name = metaReader.GetString(metaReader.GetAssemblyDefinition().Name);

Robust.Shared/ContentPack/PathHelpers.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Runtime.InteropServices;
+using Robust.Shared.Utility;
 
 namespace Robust.Shared.ContentPack
 {
@@ -63,5 +64,27 @@ namespace Robust.Shared.ContentPack
             !OperatingSystem.IsWindows()
             && !OperatingSystem.IsMacOS();
 
+
+        internal static string SafeGetResourcePath(string baseDir, ResPath path)
+        {
+            var relSysPath = path.ToRelativeSystemPath();
+            if (relSysPath.Contains("\\..") || relSysPath.Contains("/.."))
+            {
+                // Hard cap on any exploit smuggling a .. in there.
+                // Since that could allow leaving sandbox.
+                throw new InvalidOperationException($"This branch should never be reached. Path: {path}");
+            }
+
+            var retPath = Path.GetFullPath(Path.Join(baseDir, relSysPath));
+            // better safe than sorry check
+            if (!retPath.StartsWith(baseDir))
+            {
+                // Allow path to match if it's just missing the directory separator at the end.
+                if (retPath != baseDir.TrimEnd(Path.DirectorySeparatorChar))
+                    throw new InvalidOperationException($"This branch should never be reached. Path: {path}");
+            }
+
+            return retPath;
+        }
     }
 }

Robust.Shared/ContentPack/ResourceManager.cs

@@ -41,13 +41,13 @@ namespace Robust.Shared.ContentPack
         public IWritableDirProvider UserData { get; private set; } = default!;
 
         /// <inheritdoc />
-        public virtual void Initialize(string? userData)
+        public virtual void Initialize(string? userData, bool hideRootDir)
         {
             Sawmill = _logManager.GetSawmill("res");
 
             if (userData != null)
             {
-                UserData = new WritableDirProvider(Directory.CreateDirectory(userData));
+                UserData = new WritableDirProvider(Directory.CreateDirectory(userData), hideRootDir);
             }
             else
             {
@@ -379,6 +379,10 @@ namespace Robust.Shared.ContentPack
                 {
                     var rootDir = loader.GetPath(new ResPath(@"/"));
 
+                    // TODO: GET RID OF THIS.
+                    // This code shouldn't be passing OS disk paths through ResPath.
+                    rootDir = rootDir.Replace(Path.DirectorySeparatorChar, '/');
+
                     yield return new ResPath(rootDir);
                 }
             }

Robust.Shared/ContentPack/Sandbox.yml

@@ -17,6 +17,10 @@ WhitelistedNamespaces:
 - Content
 - OpenDreamShared
 
+AllowedAssemblyPrefixes:
+- OpenDream
+- Content
+
 # The type whitelist does NOT care about which assembly types come from.
 # This is because types switch assembly all the time.
 # Just look up stuff like StreamReader on https://apisof.net.
@@ -875,6 +879,7 @@ Types:
       - "System.Text.StringBuilder Insert(int, object)"
       - "System.Text.StringBuilder Insert(int, sbyte)"
       - "System.Text.StringBuilder Insert(int, short)"
+      - "System.Text.StringBuilder Insert(int, string)"
       - "System.Text.StringBuilder Insert(int, string, int)"
       - "System.Text.StringBuilder Insert(int, System.Decimal)"
       - "System.Text.StringBuilder Insert(int, System.ReadOnlySpan`1<char>)"

Robust.Shared/ContentPack/WritableDirProvider.cs

@@ -10,17 +10,22 @@ namespace Robust.Shared.ContentPack
     /// <inheritdoc />
     internal sealed class WritableDirProvider : IWritableDirProvider
     {
-        /// <inheritdoc />
+        private readonly bool _hideRootDir;
+
         public string RootDir { get; }
 
+        string? IWritableDirProvider.RootDir => _hideRootDir ? null : RootDir;
+
         /// <summary>
         /// Constructs an instance of <see cref="WritableDirProvider"/>.
         /// </summary>
         /// <param name="rootDir">Root file system directory to allow writing.</param>
-        public WritableDirProvider(DirectoryInfo rootDir)
+        /// <param name="hideRootDir">If true, <see cref="IWritableDirProvider.RootDir"/> is reported as null.</param>
+        public WritableDirProvider(DirectoryInfo rootDir, bool hideRootDir)
         {
             // FullName does not have a trailing separator, and we MUST have a separator.
             RootDir = rootDir.FullName + Path.DirectorySeparatorChar.ToString();
+            _hideRootDir = hideRootDir;
         }
 
         #region File Access
@@ -119,7 +124,7 @@ namespace Robust.Shared.ContentPack
                 throw new FileNotFoundException();
 
             var dirInfo = new DirectoryInfo(GetFullPath(path));
-            return new WritableDirProvider(dirInfo);
+            return new WritableDirProvider(dirInfo, _hideRootDir);
         }
 
         /// <inheritdoc />
@@ -180,20 +185,7 @@ namespace Robust.Shared.ContentPack
 
             path = path.Clean();
 
-            return GetFullPath(RootDir, path);
-        }
-
-        private static string GetFullPath(string root, ResPath path)
-        {
-            var relPath = path.ToRelativeSystemPath();
-            if (relPath.Contains("\\..") || relPath.Contains("/.."))
-            {
-                // Hard cap on any exploit smuggling a .. in there.
-                // Since that could allow leaving sandbox.
-                throw new InvalidOperationException($"This branch should never be reached. Path: {path}");
-            }
-
-            return Path.GetFullPath(Path.Combine(root, relPath));
+            return PathHelpers.SafeGetResourcePath(RootDir, path);
         }
     }
 }

Robust.Shared/GameObjects/ComponentFactory.cs

@@ -273,11 +273,11 @@ namespace Robust.Shared.GameObjects
 
         public IComponent GetComponent(Type componentType)
         {
-            if (!_types.ContainsKey(componentType))
+            if (!_types.TryGetValue(componentType, out var value))
             {
                 throw new InvalidOperationException($"{componentType} is not a registered component.");
             }
-            return _typeFactory.CreateInstanceUnchecked<IComponent>(_types[componentType].Type);
+            return _typeFactory.CreateInstanceUnchecked<IComponent>(value.Type);
         }
 
         public IComponent GetComponent(CompIdx componentType)
@@ -287,11 +287,11 @@ namespace Robust.Shared.GameObjects
 
         public T GetComponent<T>() where T : IComponent, new()
         {
-            if (!_types.ContainsKey(typeof(T)))
+            if (!_types.TryGetValue(typeof(T), out var reg))
             {
                 throw new InvalidOperationException($"{typeof(T)} is not a registered component.");
             }
-            return _typeFactory.CreateInstanceUnchecked<T>(_types[typeof(T)].Type);
+            return _typeFactory.CreateInstanceUnchecked<T>(reg.Type);
         }
 
         public IComponent GetComponent(ComponentRegistration reg)

Robust.Shared/GameObjects/Components/MetaDataComponent.cs

@@ -150,6 +150,7 @@ namespace Robust.Shared.GameObjects
         [ViewVariables, Access(typeof(EntityManager), Other = AccessPermissions.ReadExecute)]
         public EntityLifeStage EntityLifeStage { get; internal set; }
 
+        [ViewVariables(VVAccess.ReadOnly)]
         public MetaDataFlags Flags
         {
             get => _flags;

Robust.Shared/GameObjects/Components/Renderable/SpriteLayerData.cs

@@ -80,4 +80,8 @@ public enum LayerRenderingStrategy
     SnapToCardinals,
     NoRotation,
     UseSpriteStrategy
+    // TODO SPRITE
+    // Refactor this make the sprites strategy the actual default.
+    // That way layers have to opt in to having a custom strategy, instead of opt out.
+    // Also rename default to make it clear that its not actually the default, instead I guess its "WithRotation"?
 }

Robust.Shared/GameObjects/Components/Transform/TransformComponent.cs

@@ -167,7 +167,7 @@ namespace Robust.Shared.GameObjects
                 if (!Initialized)
                     return;
 
-                _entMan.System<SharedTransformSystem>().RaiseMoveEvent((Owner, this, meta), _parent, _localPosition, oldRotation, MapUid);
+                _entMan.System<SharedTransformSystem>().RaiseMoveEvent((Owner, this, meta), _parent, _localPosition, oldRotation, MapUid, checkTraversal: false);
             }
         }
 

Robust.Shared/GameObjects/Entity.cs

@@ -11,7 +11,7 @@ public record struct Entity<T> : IFluentEntityUid, IAsType<EntityUid>
 {
     public EntityUid Owner;
     public T Comp;
-    EntityUid IFluentEntityUid.FluentOwner => Owner;
+    readonly EntityUid IFluentEntityUid.FluentOwner => Owner;
 
     public Entity(EntityUid owner, T comp)
     {
@@ -48,9 +48,9 @@ public record struct Entity<T> : IFluentEntityUid, IAsType<EntityUid>
     }
 
 
-    public override int GetHashCode() => Owner.GetHashCode();
-    public Entity<T?> AsNullable() => new(Owner, Comp);
-    public EntityUid AsType() => Owner;
+    public override readonly int GetHashCode() => Owner.GetHashCode();
+    public readonly Entity<T?> AsNullable() => new(Owner, Comp);
+    public readonly EntityUid AsType() => Owner;
 }
 
 [NotYamlSerializable]
@@ -60,7 +60,7 @@ public record struct Entity<T1, T2> : IFluentEntityUid, IAsType<EntityUid>
     public EntityUid Owner;
     public T1 Comp1;
     public T2 Comp2;
-    EntityUid IFluentEntityUid.FluentOwner => Owner;
+    readonly EntityUid IFluentEntityUid.FluentOwner => Owner;
 
     public Entity(EntityUid owner, T1 comp1, T2 comp2)
     {
@@ -119,9 +119,9 @@ public record struct Entity<T1, T2> : IFluentEntityUid, IAsType<EntityUid>
         return new Entity<T1>(ent.Owner, ent.Comp1);
     }
 
-    public override int GetHashCode() => Owner.GetHashCode();
-    public Entity<T1?, T2?> AsNullable() => new(Owner, Comp1, Comp2);
-    public EntityUid AsType() => Owner;
+    public override readonly int GetHashCode() => Owner.GetHashCode();
+    public readonly Entity<T1?, T2?> AsNullable() => new(Owner, Comp1, Comp2);
+    public readonly EntityUid AsType() => Owner;
 }
 
 [NotYamlSerializable]
@@ -132,7 +132,7 @@ public record struct Entity<T1, T2, T3> : IFluentEntityUid, IAsType<EntityUid>
     public T1 Comp1;
     public T2 Comp2;
     public T3 Comp3;
-    EntityUid IFluentEntityUid.FluentOwner => Owner;
+    readonly EntityUid IFluentEntityUid.FluentOwner => Owner;
 
     public Entity(EntityUid owner, T1 comp1, T2 comp2, T3 comp3)
     {
@@ -226,9 +226,9 @@ public record struct Entity<T1, T2, T3> : IFluentEntityUid, IAsType<EntityUid>
 
 #endregion
 
-    public override int GetHashCode() => Owner.GetHashCode();
-    public Entity<T1?, T2?, T3?> AsNullable() => new(Owner, Comp1, Comp2, Comp3);
-    public EntityUid AsType() => Owner;
+    public override readonly int GetHashCode() => Owner.GetHashCode();
+    public readonly Entity<T1?, T2?, T3?> AsNullable() => new(Owner, Comp1, Comp2, Comp3);
+    public readonly EntityUid AsType() => Owner;
 }
 
 [NotYamlSerializable]
@@ -240,7 +240,7 @@ public record struct Entity<T1, T2, T3, T4> : IFluentEntityUid, IAsType<EntityUi
     public T2 Comp2;
     public T3 Comp3;
     public T4 Comp4;
-    EntityUid IFluentEntityUid.FluentOwner => Owner;
+    readonly EntityUid IFluentEntityUid.FluentOwner => Owner;
 
     public Entity(EntityUid owner, T1 comp1, T2 comp2, T3 comp3, T4 comp4)
     {
@@ -357,9 +357,9 @@ public record struct Entity<T1, T2, T3, T4> : IFluentEntityUid, IAsType<EntityUi
 
 #endregion
 
-    public override int GetHashCode() => Owner.GetHashCode();
-    public Entity<T1?, T2?, T3?, T4?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4);
-    public EntityUid AsType() => Owner;
+    public override readonly int GetHashCode() => Owner.GetHashCode();
+    public readonly Entity<T1?, T2?, T3?, T4?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4);
+    public readonly EntityUid AsType() => Owner;
 }
 
 [NotYamlSerializable]
@@ -372,7 +372,7 @@ public record struct Entity<T1, T2, T3, T4, T5> : IFluentEntityUid, IAsType<Enti
     public T3 Comp3;
     public T4 Comp4;
     public T5 Comp5;
-    EntityUid IFluentEntityUid.FluentOwner => Owner;
+    readonly EntityUid IFluentEntityUid.FluentOwner => Owner;
 
     public Entity(EntityUid owner, T1 comp1, T2 comp2, T3 comp3, T4 comp4, T5 comp5)
     {
@@ -512,9 +512,9 @@ public record struct Entity<T1, T2, T3, T4, T5> : IFluentEntityUid, IAsType<Enti
 
 #endregion
 
-    public override int GetHashCode() => Owner.GetHashCode();
-    public Entity<T1?, T2?, T3?, T4?, T5?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4, Comp5);
-    public EntityUid AsType() => Owner;
+    public override readonly int GetHashCode() => Owner.GetHashCode();
+    public readonly Entity<T1?, T2?, T3?, T4?, T5?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4, Comp5);
+    public readonly EntityUid AsType() => Owner;
 }
 
 [NotYamlSerializable]
@@ -528,7 +528,7 @@ public record struct Entity<T1, T2, T3, T4, T5, T6> : IFluentEntityUid, IAsType<
     public T4 Comp4;
     public T5 Comp5;
     public T6 Comp6;
-    EntityUid IFluentEntityUid.FluentOwner => Owner;
+    readonly EntityUid IFluentEntityUid.FluentOwner => Owner;
 
     public Entity(EntityUid owner, T1 comp1, T2 comp2, T3 comp3, T4 comp4, T5 comp5, T6 comp6)
     {
@@ -691,9 +691,9 @@ public record struct Entity<T1, T2, T3, T4, T5, T6> : IFluentEntityUid, IAsType<
 
 #endregion
 
-    public override int GetHashCode() => Owner.GetHashCode();
-    public Entity<T1?, T2?, T3?, T4?, T5?, T6?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4, Comp5, Comp6);
-    public EntityUid AsType() => Owner;
+    public override readonly int GetHashCode() => Owner.GetHashCode();
+    public readonly Entity<T1?, T2?, T3?, T4?, T5?, T6?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4, Comp5, Comp6);
+    public readonly EntityUid AsType() => Owner;
 }
 
 [NotYamlSerializable]
@@ -708,7 +708,7 @@ public record struct Entity<T1, T2, T3, T4, T5, T6, T7> : IFluentEntityUid, IAsT
     public T5 Comp5;
     public T6 Comp6;
     public T7 Comp7;
-    EntityUid IFluentEntityUid.FluentOwner => Owner;
+    readonly EntityUid IFluentEntityUid.FluentOwner => Owner;
 
     public Entity(EntityUid owner, T1 comp1, T2 comp2, T3 comp3, T4 comp4, T5 comp5, T6 comp6, T7 comp7)
     {
@@ -894,9 +894,9 @@ public record struct Entity<T1, T2, T3, T4, T5, T6, T7> : IFluentEntityUid, IAsT
 
 #endregion
 
-    public override int GetHashCode() => Owner.GetHashCode();
-    public Entity<T1?, T2?, T3?, T4?, T5?, T6?, T7?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4, Comp5, Comp6, Comp7);
-    public EntityUid AsType() => Owner;
+    public override readonly int GetHashCode() => Owner.GetHashCode();
+    public readonly Entity<T1?, T2?, T3?, T4?, T5?, T6?, T7?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4, Comp5, Comp6, Comp7);
+    public readonly EntityUid AsType() => Owner;
 }
 
 [NotYamlSerializable]
@@ -912,7 +912,7 @@ public record struct Entity<T1, T2, T3, T4, T5, T6, T7, T8> : IFluentEntityUid,
     public T6 Comp6;
     public T7 Comp7;
     public T8 Comp8;
-    EntityUid IFluentEntityUid.FluentOwner => Owner;
+    readonly EntityUid IFluentEntityUid.FluentOwner => Owner;
 
     public Entity(EntityUid owner, T1 comp1, T2 comp2, T3 comp3, T4 comp4, T5 comp5, T6 comp6, T7 comp7, T8 comp8)
     {
@@ -1121,7 +1121,7 @@ public record struct Entity<T1, T2, T3, T4, T5, T6, T7, T8> : IFluentEntityUid,
 
 #endregion
 
-    public override int GetHashCode() => Owner.GetHashCode();
-    public Entity<T1?, T2?, T3?, T4?, T5?, T6?, T7?, T8?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4, Comp5, Comp6, Comp7, Comp8);
-    public EntityUid AsType() => Owner;
+    public override readonly int GetHashCode() => Owner.GetHashCode();
+    public readonly Entity<T1?, T2?, T3?, T4?, T5?, T6?, T7?, T8?> AsNullable() => new(Owner, Comp1, Comp2, Comp3, Comp4, Comp5, Comp6, Comp7, Comp8);
+    public readonly EntityUid AsType() => Owner;
 }

Robust.Shared/GameObjects/EntityManager.Spawn.cs

@@ -52,6 +52,16 @@ public partial class EntityManager
         return ents;
     }
 
+    public EntityUid[] SpawnEntitiesAttachedTo(EntityCoordinates coordinates, params EntProtoId[] protoNames)
+    {
+        var ents = new EntityUid[protoNames.Length];
+        for (var i = 0; i < protoNames.Length; i++)
+        {
+            ents[i] = SpawnAttachedTo(protoNames[i], coordinates);
+        }
+        return ents;
+    }
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public EntityUid[] SpawnEntitiesAttachedTo(EntityCoordinates coordinates, List<string?> protoNames)
     {
@@ -74,6 +84,14 @@ public partial class EntityManager
         return ents;
     }
 
+    public void SpawnEntitiesAttachedTo(EntityCoordinates coordinates, IEnumerable<EntProtoId> protoNames)
+    {
+        foreach (var protoName in protoNames)
+        {
+            SpawnAttachedTo(protoName, coordinates);
+        }
+    }
+
     public virtual EntityUid SpawnAttachedTo(string? protoName, EntityCoordinates coordinates, ComponentRegistry? overrides = null, Angle rotation = default)
     {
         if (!coordinates.IsValid(this))

Robust.Shared/GameObjects/EntitySystem.cs

@@ -29,6 +29,8 @@ namespace Robust.Shared.GameObjects
         [Dependency] private readonly IReplayRecordingManager _replayMan = default!;
         [Dependency] protected readonly ILocalizationManager Loc = default!;
 
+        protected IComponentFactory Factory => EntityManager.ComponentFactory;
+
         public ISawmill Log { get; private set; } = default!;
 
         protected virtual string SawmillName

Robust.Shared/GameObjects/IEntityManager.Spawn.cs

@@ -22,6 +22,8 @@ public partial interface IEntityManager
     EntityUid[] SpawnEntities(MapCoordinates coordinates, params string?[] protoNames);
     EntityUid[] SpawnEntities(MapCoordinates coordinates, string? prototype, int count);
     EntityUid[] SpawnEntities(MapCoordinates coordinates, List<string?> protoNames);
+    void SpawnEntitiesAttachedTo(EntityCoordinates coordinates, IEnumerable<EntProtoId> protoNames);
+    EntityUid[] SpawnEntitiesAttachedTo(EntityCoordinates coordinates, params EntProtoId[] protoNames);
     EntityUid[] SpawnEntitiesAttachedTo(EntityCoordinates coordinates, List<string?> protoNames);
     EntityUid[] SpawnEntitiesAttachedTo(EntityCoordinates coordinates, params string?[] protoNames);
 

Robust.Shared/GameObjects/Systems/SharedGridTraversalSystem.cs

@@ -88,6 +88,7 @@ public sealed class SharedGridTraversalSystem : EntitySystem
 
     public void CheckTraversal(EntityUid entity, TransformComponent xform, EntityUid map)
     {
+        // TODO: This should probably check center of mass.
         DebugTools.Assert(!HasComp<MapGridComponent>(entity));
         DebugTools.Assert(!HasComp<MapComponent>(entity));
 

Robust.Shared/GameObjects/Systems/SharedMapSystem.Grid.cs

@@ -4,6 +4,7 @@ using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Numerics;
 using System.Runtime.CompilerServices;
+using Robust.Shared.Collections;
 using Robust.Shared.GameStates;
 using Robust.Shared.Map;
 using Robust.Shared.Map.Components;
@@ -833,7 +834,7 @@ public abstract partial class SharedMapSystem
         }
 
         var offset = chunk.GridTileToChunkTile(gridIndices);
-        SetChunkTile(uid, grid, chunk, (ushort)offset.X, (ushort)offset.Y, tile);
+        SetChunkTile(uid, grid, chunk, (ushort)offset.X, (ushort)offset.Y, tile, out _);
     }
 
     public void SetTiles(EntityUid uid, MapGridComponent grid, List<(Vector2i GridIndices, Tile Tile)> tiles)
@@ -842,6 +843,11 @@ public abstract partial class SharedMapSystem
             return;
 
         var modified = new HashSet<MapChunk>(Math.Max(1, tiles.Count / grid.ChunkSize));
+        var tileChanges = new ValueList<TileChangedEntry>(tiles.Count);
+
+        // Suppress sending out events for each tile changed
+        // We're going to send them all out together at the end
+        MapManager.SuppressOnTileChanged = true;
 
         foreach (var (gridIndices, tile) in tiles)
         {
@@ -859,8 +865,11 @@ public abstract partial class SharedMapSystem
 
             var offset = chunk.GridTileToChunkTile(gridIndices);
             chunk.SuppressCollisionRegeneration = true;
-            if (SetChunkTile(uid, grid, chunk, (ushort)offset.X, (ushort)offset.Y, tile))
+            if (SetChunkTile(uid, grid, chunk, (ushort)offset.X, (ushort)offset.Y, tile, out var oldTile))
+            {
                 modified.Add(chunk);
+                tileChanges.Add(new TileChangedEntry(tile, oldTile, offset, gridIndices));
+            }
         }
 
         foreach (var chunk in modified)
@@ -869,6 +878,13 @@ public abstract partial class SharedMapSystem
         }
 
         RegenerateCollision(uid, grid, modified);
+
+        // Notify of all tile changes in one event
+        var ev = new TileChangedEvent((uid, grid), tileChanges.ToArray());
+        RaiseLocalEvent(uid, ref ev, true);
+
+        // Back to normal
+        MapManager.SuppressOnTileChanged = false;
     }
 
     public TilesEnumerator GetLocalTilesEnumerator(EntityUid uid, MapGridComponent grid, Box2 aabb,

Robust.Shared/GameObjects/Systems/SharedMapSystem.GridChunk.cs

@@ -14,9 +14,9 @@ public abstract partial class SharedMapSystem
     /// <param name="xIndex">The X tile index relative to the chunk.</param>
     /// <param name="yIndex">The Y tile index relative to the chunk.</param>
     /// <param name="tile">The new tile to insert.</param>
-    internal bool SetChunkTile(EntityUid uid, MapGridComponent grid, MapChunk chunk, ushort xIndex, ushort yIndex, Tile tile)
+    internal bool SetChunkTile(EntityUid uid, MapGridComponent grid, MapChunk chunk, ushort xIndex, ushort yIndex, Tile tile, out Tile oldTile)
     {
-        if (!chunk.TrySetTile(xIndex, yIndex, tile, out var oldTile, out var shapeChanged))
+        if (!chunk.TrySetTile(xIndex, yIndex, tile, out oldTile, out var shapeChanged))
             return false;
 
         var tileIndices = new Vector2i(xIndex, yIndex);

Robust.Shared/GameObjects/Systems/SharedMapSystem.cs

@@ -173,45 +173,61 @@ namespace Robust.Shared.GameObjects
     }
 
     /// <summary>
-    ///     Arguments for when a single tile on a grid is changed locally or remotely.
+    /// Raised directed at the grid when tiles are changed locally or remotely.
     /// </summary>
     [ByRefEvent]
     public readonly record struct TileChangedEvent
     {
-        /// <summary>
-        ///     Creates a new instance of this class.
-        /// </summary>
+        /// <inheritdoc cref="TileChangedEvent(Entity{MapGridComponent}, Tile, Tile, Vector2i, Vector2i)"/>
         public TileChangedEvent(Entity<MapGridComponent> entity, TileRef newTile, Tile oldTile, Vector2i chunkIndex)
+            : this(entity, newTile.Tile, oldTile, chunkIndex, newTile.GridIndices) { }
+
+        /// <summary>
+        /// Creates a new instance of this event for a single changed tile.
+        /// </summary>
+        /// <param name="entity">The grid entity containing the changed tile(s)</param>
+        /// <param name="newTile">New tile that replaced the old one.</param>
+        /// <param name="oldTile">Old tile that was replaced.</param>
+        /// <param name="chunkIndex">The index of the grid-chunk that this tile belongs to.</param>
+        /// <param name="gridIndices">The positional indices of this tile on the grid.</param>
+        public TileChangedEvent(Entity<MapGridComponent> entity, Tile newTile, Tile oldTile, Vector2i chunkIndex, Vector2i gridIndices)
         {
             Entity = entity;
-            NewTile = newTile;
-            OldTile = oldTile;
-            ChunkIndex = chunkIndex;
+            Changes = [new TileChangedEntry(newTile, oldTile, chunkIndex, gridIndices)];
         }
 
         /// <summary>
-        /// Was the tile previously empty or is it now empty.
+        /// Creates a new instance of this event for multiple changed tiles.
         /// </summary>
-        public bool EmptyChanged => OldTile.IsEmpty != NewTile.Tile.IsEmpty;
+        public TileChangedEvent(Entity<MapGridComponent> entity, TileChangedEntry[] changes)
+        {
+            Entity = entity;
+            Changes = changes;
+        }
 
         /// <summary>
-        ///     Entity of the grid with the tile-change. TileRef stores the GridId.
+        /// Entity of the grid with the tile-change. TileRef stores the GridId.
         /// </summary>
         public readonly Entity<MapGridComponent> Entity;
 
         /// <summary>
-        ///     New tile that replaced the old one.
+        /// An array of all the tiles that were changed.
         /// </summary>
-        public readonly TileRef NewTile;
+        public readonly TileChangedEntry[] Changes;
+    }
 
+    /// <summary>
+    /// Data about a single tile that was changed as part of a <see cref="TileChangedEvent"/>.
+    /// </summary>
+    /// <param name="NewTile">New tile that replaced the old one.</param>
+    /// <param name="OldTile">Old tile that was replaced.</param>
+    /// <param name="ChunkIndex">The index of the grid-chunk that this tile belongs to.</param>
+    /// <param name="GridIndices">The positional indices of this tile on the grid.</param>
+    public readonly record struct TileChangedEntry(Tile NewTile, Tile OldTile, Vector2i ChunkIndex, Vector2i GridIndices)
+    {
         /// <summary>
-        ///     Old tile that was replaced.
+        /// Was the tile previously empty or is it now empty.
         /// </summary>
-        public readonly Tile OldTile;
-
-        /// <summary>
-        ///     The index of the grid-chunk that this tile belongs to.
-        /// </summary>
-        public readonly Vector2i ChunkIndex;
+        public bool EmptyChanged => OldTile.IsEmpty != NewTile.IsEmpty;
     }
 }

Robust.Shared/GameObjects/Systems/SharedTransformSystem.Component.cs

@@ -651,19 +651,30 @@ public abstract partial class SharedTransformSystem
         if (newMapId == ent.Comp1.MapID)
             return;
 
-        EntityUid? newMap = newMapId == MapId.Nullspace ? null : _map.GetMap(newMapId);
-        var mapPaused = _map.IsPaused(newMapId);
+        EntityUid? newUid = newMapId == MapId.Nullspace ? null : _map.GetMap(newMapId);
+        bool? mapPaused = null;
 
-        ChangeMapIdRecursive(ent, newMap, newMapId, mapPaused);
+        // Client may be moving entities across maps due to things leaving or entering PVS range.
+        // In that case, we don't want to pause or unpause entities.
+        if (!_gameTiming.ApplyingState)
+        {
+            mapPaused = _map.IsPaused(newMapId);
+            _metaData.SetEntityPaused(ent.Owner, mapPaused.Value, ent.Comp2);
+        }
+
+        ChangeMapIdRecursive(ent, newUid, newMapId, mapPaused);
     }
 
     private void ChangeMapIdRecursive(
         Entity<TransformComponent, MetaDataComponent> ent,
         EntityUid? newMap,
         MapId newMapId,
-        bool paused)
+        bool? paused)
     {
-        _metaData.SetEntityPaused(ent.Owner, paused, ent.Comp2);
+        if (paused is { } p)
+        {
+            _metaData.SetEntityPaused(ent.Owner, p, ent.Comp2);
+        }
 
         if ((ent.Comp2.Flags & MetaDataFlags.ExtraTransformEvents) != 0)
         {
@@ -1258,7 +1269,13 @@ public abstract partial class SharedTransformSystem
         if (!xform.Initialized)
             return;
 
-        RaiseMoveEvent((uid, xform, meta), oldParent, oldPosition, oldRotation, xform.MapUid);
+        RaiseMoveEvent(
+            (uid, xform, meta),
+            oldParent,
+            oldPosition,
+            oldRotation,
+            xform.MapUid,
+            checkTraversal: !oldPosition.Equals(pos));
     }
 
     #endregion

Robust.Shared/GameObjects/Systems/SharedTransformSystem.cs

@@ -70,11 +70,14 @@ namespace Robust.Shared.GameObjects
 
         private void MapManagerOnTileChanged(ref TileChangedEvent e)
         {
-            if(e.NewTile.Tile != Tile.Empty)
-                return;
+            foreach (var change in e.Changes)
+            {
+                if(change.NewTile != Tile.Empty)
+                    continue;
 
-            // TODO optimize this for when multiple tiles get empties simultaneously (e.g., explosions).
-            DeparentAllEntsOnTile(e.NewTile.GridUid, e.NewTile.GridIndices);
+                // TODO optimize this for when multiple tiles get empties simultaneously (e.g., explosions).
+                DeparentAllEntsOnTile(e.Entity, change.GridIndices);
+            }
         }
 
         /// <summary>
@@ -265,7 +268,8 @@ namespace Robust.Shared.GameObjects
             EntityUid oldParent,
             Vector2 oldPosition,
             Angle oldRotation,
-            EntityUid? oldMap)
+            EntityUid? oldMap,
+            bool checkTraversal = true)
         {
             var pos = ent.Comp1._parent == EntityUid.Invalid
                 ? default
@@ -295,7 +299,10 @@ namespace Robust.Shared.GameObjects
             // Finally, handle grid traversal. This is handled separately to avoid out-of-order move events.
             // I.e., if the traversal raises its own move event, this ensures that all the old move event handlers
             // have finished running first. Ideally this shouldn't be required, but this is here just in case
-            _traversal.CheckTraverse(ent);
+            if (checkTraversal)
+            {
+                _traversal.CheckTraverse(ent);
+            }
         }
     }
 

Robust.Shared/Physics/Events/StartCollideEvent.cs

@@ -1,8 +1,8 @@
 using System.Numerics;
 using Robust.Shared.GameObjects;
-using Robust.Shared.Maths;
 using Robust.Shared.Physics.Components;
 using Robust.Shared.Physics.Dynamics;
+using Robust.Shared.Utility;
 
 namespace Robust.Shared.Physics.Events;
 
@@ -20,9 +20,16 @@ public readonly struct StartCollideEvent
 
     public readonly Fixture OurFixture;
     public readonly Fixture OtherFixture;
-    public readonly Vector2 WorldPoint;
 
-    public StartCollideEvent(
+    internal readonly FixedArray2<Vector2> _worldPoints;
+
+    public readonly int PointCount;
+    public readonly Vector2 WorldNormal;
+
+    public Vector2[] WorldPoints => _worldPoints.AsSpan[..PointCount].ToArray();
+
+
+    internal StartCollideEvent(
         EntityUid ourEntity,
         EntityUid otherEntity,
         string ourFixtureId,
@@ -31,7 +38,9 @@ public readonly struct StartCollideEvent
         Fixture otherFixture,
         PhysicsComponent ourBody,
         PhysicsComponent otherBody,
-        Vector2 worldPoint)
+        FixedArray2<Vector2> worldPoints,
+        int pointCount,
+        Vector2 worldNormal)
     {
         OurEntity = ourEntity;
         OtherEntity = otherEntity;
@@ -39,8 +48,10 @@ public readonly struct StartCollideEvent
         OtherFixtureId = otherFixtureId;
         OurFixture = ourFixture;
         OtherFixture = otherFixture;
-        WorldPoint = worldPoint;
         OtherBody = otherBody;
         OurBody = ourBody;
+        _worldPoints = worldPoints;
+        PointCount = pointCount;
+        WorldNormal = worldNormal;
     }
 }

Robust.Shared/Physics/Systems/SharedPhysicsSystem.Contacts.cs

@@ -294,6 +294,14 @@ public abstract partial class SharedPhysicsSystem
         DebugTools.Assert(!fixB.Contacts.ContainsKey(fixA));
         fixB.Contacts.Add(fixA, contact);
         bodB.Contacts.AddLast(contact.BodyBNode);
+
+        // If it's a spawned static ent then need to wake any contacting entities.
+        // The issue is that static ents can never be awake and if it spawns on an asleep entity never gets a contact.
+        // Checking only bodyA should be okay because if bodyA is the other ent (i.e. dynamic / kinematic) then it should already be awake.
+        if (bodyA.BodyType == BodyType.Static && !bodyB.Awake)
+        {
+            WakeBody(uidB, body: bodyB);
+        }
     }
 
     /// <summary>
@@ -350,10 +358,10 @@ public abstract partial class SharedPhysicsSystem
 
         if (contact.Manifold.PointCount > 0 && contact.FixtureA?.Hard == true && contact.FixtureB?.Hard == true)
         {
-            if (bodyA.CanCollide)
+            if (bodyA.CanCollide && !TerminatingOrDeleted(aUid))
                 SetAwake((aUid, bodyA), true);
 
-            if (bodyB.CanCollide)
+            if (bodyB.CanCollide && !TerminatingOrDeleted(bUid))
                 SetAwake((bUid, bodyB), true);
         }
 
@@ -543,7 +551,7 @@ public abstract partial class SharedPhysicsSystem
         }
 
         var status = ArrayPool<ContactStatus>.Shared.Rent(index);
-        var worldPoints = ArrayPool<Vector2>.Shared.Rent(index);
+        var worldPoints = ArrayPool<FixedArray4<Vector2>>.Shared.Rent(index);
 
         // Update contacts all at once.
         BuildManifolds(contacts, index, status, worldPoints);
@@ -578,9 +586,11 @@ public abstract partial class SharedPhysicsSystem
                     var uidA = contact.EntityA;
                     var uidB = contact.EntityB;
                     var worldPoint = worldPoints[i];
+                    var points = new FixedArray2<Vector2>(worldPoint._00, worldPoint._01);
+                    var worldNormal = worldPoint._02;
 
-                    var ev1 = new StartCollideEvent(uidA, uidB, contact.FixtureAId, contact.FixtureBId, fixtureA, fixtureB, bodyA, bodyB, worldPoint);
-                    var ev2 = new StartCollideEvent(uidB, uidA, contact.FixtureBId, contact.FixtureAId, fixtureB, fixtureA, bodyB, bodyA, worldPoint);
+                    var ev1 = new StartCollideEvent(uidA, uidB, contact.FixtureAId, contact.FixtureBId, fixtureA, fixtureB, bodyA, bodyB, points, contact.Manifold.PointCount, worldNormal);
+                    var ev2 = new StartCollideEvent(uidB, uidA, contact.FixtureBId, contact.FixtureAId, fixtureB, fixtureA, bodyB, bodyA, points, contact.Manifold.PointCount, worldNormal);
 
                     RaiseLocalEvent(uidA, ref ev1, true);
                     RaiseLocalEvent(uidB, ref ev2, true);
@@ -618,10 +628,10 @@ public abstract partial class SharedPhysicsSystem
 
         ArrayPool<Contact>.Shared.Return(contacts);
         ArrayPool<ContactStatus>.Shared.Return(status);
-        ArrayPool<Vector2>.Shared.Return(worldPoints);
+        ArrayPool<FixedArray4<Vector2>>.Shared.Return(worldPoints);
     }
 
-    private void BuildManifolds(Contact[] contacts, int count, ContactStatus[] status, Vector2[] worldPoints)
+    private void BuildManifolds(Contact[] contacts, int count, ContactStatus[] status, FixedArray4<Vector2>[] worldPoints)
     {
         if (count == 0)
             return;
@@ -664,7 +674,7 @@ public abstract partial class SharedPhysicsSystem
 
         public Contact[] Contacts;
         public ContactStatus[] Status;
-        public Vector2[] WorldPoints;
+        public FixedArray4<Vector2>[] WorldPoints;
         public bool[] Wake;
 
         public void Execute(int index)
@@ -673,7 +683,7 @@ public abstract partial class SharedPhysicsSystem
         }
     }
 
-    private void UpdateContact(Contact[] contacts, int index, ContactStatus[] status, bool[] wake, Vector2[] worldPoints)
+    private void UpdateContact(Contact[] contacts, int index, ContactStatus[] status, bool[] wake, FixedArray4<Vector2>[] worldPoints)
     {
         var contact = contacts[index];
 
@@ -698,7 +708,11 @@ public abstract partial class SharedPhysicsSystem
 
         if (contactStatus == ContactStatus.StartTouching)
         {
-            worldPoints[index] = Physics.Transform.Mul(bodyATransform, contacts[index].Manifold.LocalPoint);
+            var points = new FixedArray4<Vector2>();
+            contact.GetWorldManifold(bodyATransform, bodyBTransform, out var worldNormal, points.AsSpan);
+            // Use the 3rd Vector2 as the world normal, 4th is blank.
+            points._02 = worldNormal;
+            worldPoints[index] = points;
         }
     }
 

Robust.Shared/Properties/AssemblyInfo.cs

@@ -9,7 +9,6 @@
 [assembly: InternalsVisibleTo("Robust.UnitTesting")]
 [assembly: InternalsVisibleTo("OpenToolkit.GraphicsLibraryFramework")]
 [assembly: InternalsVisibleTo("DynamicProxyGenAssembly2")] // Gives access to Castle(Moq)
-[assembly: InternalsVisibleTo("Content.Benchmarks")]
 [assembly: InternalsVisibleTo("Robust.Benchmarks")]
 [assembly: InternalsVisibleTo("Robust.Client.WebView")]
 [assembly: InternalsVisibleTo("Robust.Packaging")]

Robust.Shared/Prototypes/IPrototypeManager.cs

@@ -306,6 +306,11 @@ public interface IPrototypeManager
     /// </summary>
     void RegisterIgnore(string name);
 
+    /// <summary>
+    /// Checks whether the given gind name has been marked as ignored via <see cref="RegisterIgnore"/>
+    /// </summary>
+    bool IsIgnored(string name);
+
     /// <summary>
     /// Loads several prototype kinds into the manager. Note that this will re-build a frozen dictionary and should be avoided if possible.
     /// </summary>

Robust.Shared/Prototypes/PrototypeManager.cs

@@ -920,6 +920,8 @@ namespace Robust.Shared.Prototypes
             return TryGetKindFrom(typeof(T), out kind);
         }
 
+        public bool IsIgnored(string name) => _ignoredPrototypeTypes.Contains(name);
+
         /// <inheritdoc />
         public void RegisterIgnore(string name)
         {
@@ -973,6 +975,17 @@ namespace Robust.Shared.Prototypes
 
             var name = attribute.Type ?? CalculatePrototypeName(kind);
 
+            if (_ignoredPrototypeTypes.Contains(name))
+            {
+                // For whatever reason, we are registering a prototype despite it having been marked as ignored.
+                // This often happens when someone is moving a server or client prototype to shared. Maybe this should
+                // log an error, but I want to avoid breaking changes and maaaaybe there some weird instance where you
+                // want the client to know that a prototype kind exists, without having the client load information
+                // about the individual prototypes? So for now lets just log a warning instead of introducing breaking
+                // changes.
+                Sawmill.Warning($"Registering an ignored prototype {kind}");
+            }
+
             if (_kindNames.TryGetValue(name, out var existing))
             {
                 throw new InvalidImplementationException(kind,

Robust.Shared/Serialization/NetBitArraySerializer.cs

@@ -0,0 +1,78 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.IO;
+using System.Reflection;
+using System.Runtime.Serialization;
+using JetBrains.Annotations;
+using NetSerializer;
+
+namespace Robust.Shared.Serialization;
+
+/// <summary>
+/// Custom serializer implementation for <see cref="BitArray"/>.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This type is necessary as, since .NET 10, the internal layout of <see cref="BitArray"/> was changed.
+/// The type now (internally) implements <see cref="ISerializable"/> for backwards compatibility with existing
+/// <c>BinaryFormatter</c> code, but NetSerializer does not support <see cref="ISerializable"/>.
+/// </para>
+/// <para>
+/// This code is designed to be backportable &amp; network compatible with the previous behavior on .NET 9.
+/// </para>
+/// </remarks>
+internal sealed class NetBitArraySerializer : IStaticTypeSerializer
+{
+    // For reference, the layout of BitArray before .NET 10 was:
+    // private int[] m_array;
+    // private int m_length;
+    // private int _version;
+    // NetSerializer serialized these in the following order (sorted by name):
+    // _version, m_array, m_length
+
+    public bool Handles(Type type)
+    {
+        return type == typeof(BitArray);
+    }
+
+    public IEnumerable<Type> GetSubtypes(Type type)
+    {
+        return [typeof(int[]), typeof(int)];
+    }
+
+    public MethodInfo GetStaticWriter(Type type)
+    {
+        return typeof(NetBitArraySerializer).GetMethod("Write", BindingFlags.Static | BindingFlags.NonPublic)!;
+    }
+
+    public MethodInfo GetStaticReader(Type type)
+    {
+        return typeof(NetBitArraySerializer).GetMethod("Read", BindingFlags.Static | BindingFlags.NonPublic)!;
+    }
+
+    [UsedImplicitly]
+    private static void Write(Serializer serializer, Stream stream, BitArray value)
+    {
+        var intCount = (31 + value.Length) >> 5;
+        var ints = new int[intCount];
+        value.CopyTo(ints, 0);
+
+        serializer.SerializeDirect(stream, 0); // _version
+        serializer.SerializeDirect(stream, ints); // m_array
+        serializer.SerializeDirect(stream, value.Length); // m_length
+    }
+
+    [UsedImplicitly]
+    private static void Read(Serializer serializer, Stream stream, out BitArray value)
+    {
+        serializer.DeserializeDirect<int>(stream, out _); // _version
+        serializer.DeserializeDirect<int[]>(stream, out var array); // m_array
+        serializer.DeserializeDirect<int>(stream, out var length); // m_length
+
+        value = new BitArray(array)
+        {
+            Length = length
+        };
+    }
+}

Robust.Shared/Serialization/RobustSerializer.cs

@@ -91,6 +91,7 @@ namespace Robust.Shared.Serialization
                     MappedStringSerializer.TypeSerializer,
                     new Vector2Serializer(),
                     new Matrix3x2Serializer(),
+                    new NetBitArraySerializer()
                 }
             };
             _serializer = new Serializer(types, settings);

Robust.Shared/Serialization/TypeSerializers/Implementations/Custom/Prototype/PrototypeIdSerializer.cs

@@ -3,6 +3,7 @@ using Robust.Shared.Prototypes;
 using Robust.Shared.Serialization.Manager;
 using Robust.Shared.Serialization.Markdown.Validation;
 using Robust.Shared.Serialization.Markdown.Value;
+using Robust.Shared.Serialization.TypeSerializers.Implementations.Generic;
 using Robust.Shared.Serialization.TypeSerializers.Interfaces;
 
 namespace Robust.Shared.Serialization.TypeSerializers.Implementations.Custom.Prototype
@@ -30,10 +31,7 @@ namespace Robust.Shared.Serialization.TypeSerializers.Implementations.Custom.Pro
         public virtual ValidationNode Validate(ISerializationManager serializationManager, ValueDataNode node,
             IDependencyCollection dependencies, ISerializationContext? context = null)
         {
-            var protoMan = dependencies.Resolve<IPrototypeManager>();
-            return protoMan.TryGetKindFrom<TPrototype>(out _) && protoMan.HasIndex<TPrototype>(node.Value)
-                ? new ValidatedValueNode(node)
-                : new ErrorNode(node, $"PrototypeID {node.Value} for type {typeof(TPrototype)} at {node.Start} not found");
+            return ProtoIdSerializer<TPrototype>.Validate(dependencies, node);
         }
     }
 }

Robust.Shared/Serialization/TypeSerializers/Implementations/Generic/ProtoIdSerializer.cs

@@ -19,8 +19,19 @@ public sealed class ProtoIdSerializer<T> : ITypeSerializer<ProtoId<T>, ValueData
 {
     public ValidationNode Validate(ISerializationManager serialization, ValueDataNode node, IDependencyCollection dependencies, ISerializationContext? context = null)
     {
-        var prototypes = dependencies.Resolve<IPrototypeManager>();
-        if (prototypes.TryGetKindFrom<T>(out _) && prototypes.HasMapping<T>(node.Value))
+        return Validate(dependencies, node);
+    }
+
+    public static ValidationNode Validate(IDependencyCollection deps, ValueDataNode node)
+    {
+        var proto = deps.Resolve<IPrototypeManager>();
+        if (!proto.TryGetKindFrom<T>(out var kind))
+            return new ErrorNode(node, $"Unknown prototype kind: {typeof(T)}");
+
+        if (proto.IsIgnored(kind))
+            return new ErrorNode(node,$"Attempting to validate an ignored prototype: {typeof(T)}.\nDid you forget to remove the IPrototypeManager.RegisterIgnore(\"{kind}\") call when moving a prototype to Shared?");
+
+        if (proto.HasMapping<T>(node.Value))
             return new ValidatedValueNode(node);
 
         return new ErrorNode(node, $"No {typeof(T)} found with id {node.Value}");

Robust.Shared/Serialization/TypeSerializers/Implementations/SpriteSpecifierSerializer.cs

@@ -25,7 +25,7 @@ namespace Robust.Shared.Serialization.TypeSerializers.Implementations
         ITypeCopier<Rsi>,
         ITypeCopier<Texture>
     {
-        // Should probably be in SpriteComponent, but is needed for server to validate paths.
+        // Should probably be in SpriteSystem, but is needed for server to validate paths.
         // So I guess it might as well go here?
         public static readonly ResPath TextureRoot = new("/Textures");
 

Robust.Shared/Utility/ResPath.cs

@@ -66,7 +66,7 @@ public readonly struct ResPath : IEquatable<ResPath>
     public ResPath(string canonPath)
     {
         // Paths should never have non-standardised directory separators passed in, the caller should have already sanitised it.
-        DebugTools.Assert(!canonPath.Contains('\\'));
+        DebugTools.Assert(IsValidPath(canonPath));
         CanonPath = canonPath;
     }
 
@@ -77,6 +77,21 @@ public readonly struct ResPath : IEquatable<ResPath>
     {
     }
 
+    /// <summary>
+    /// Check whether the given string paths contains any non-standard directory separators.
+    /// </summary>
+    public static bool IsValidPath(string path) => !path.Contains('\\');
+
+    /// <summary>
+    /// Check whether a string is a valid path (<see cref="IsValidPath"/>) and corresponds to a simple file name.
+    /// </summary>
+    public static bool IsValidFilename([NotNullWhen(true)] string? filename)
+        => !string.IsNullOrEmpty(filename)
+           && IsValidPath(filename)
+           && !filename.Contains('/')
+           && filename != "."
+           && filename != "..";
+
     /// <summary>
     ///     Returns true if the path is equal to "."
     /// </summary>
@@ -106,7 +121,7 @@ public readonly struct ResPath : IEquatable<ResPath>
             }
 
             var ind = CanonPath.Length > 1 && CanonPath[^1] == '/'
-                ? CanonPath[..^1].LastIndexOf('/')
+                ? CanonPath.LastIndexOf('/', CanonPath.Length - 2)
                 : CanonPath.LastIndexOf('/');
             return ind switch
             {
@@ -206,7 +221,7 @@ public readonly struct ResPath : IEquatable<ResPath>
             // it's a filename
             // Uses +1 to skip `/` found in or starts from beginning of string
             // if we found nothing (ind == -1)
-            var ind = CanonPath[..^1].LastIndexOf('/') + 1;
+            var ind = CanonPath.LastIndexOf('/', CanonPath.Length - 2) + 1;
             return CanonPath[^1] == '/'
                 ? CanonPath[ind .. ^1] // Omit last `/`
                 : CanonPath[ind..];
@@ -242,7 +257,6 @@ public readonly struct ResPath : IEquatable<ResPath>
         return CanonPath.GetHashCode();
     }
 
-
     public static bool operator ==(ResPath left, ResPath right)
     {
         return left.Equals(right);
@@ -283,7 +297,7 @@ public readonly struct ResPath : IEquatable<ResPath>
         }
 
         // Avoid double separators
-        if (left.CanonPath.EndsWith("/"))
+        if (left.CanonPath.EndsWith('/'))
         {
             return new ResPath(left.CanonPath + right.CanonPath);
         }
@@ -475,7 +489,7 @@ public readonly struct ResPath : IEquatable<ResPath>
     /// </summary>
     public string ToRelativeSystemPath()
     {
-        return ToRelativePath().ChangeSeparator(SystemSeparatorStr);
+        return ToRelativePath().ChangeSeparator(SystemSeparator);
     }
 
     /// <summary>
@@ -495,14 +509,19 @@ public readonly struct ResPath : IEquatable<ResPath>
     /// </summary>
     public string ChangeSeparator(string newSeparator)
     {
-        if (newSeparator is "." or "\0")
-        {
-            throw new ArgumentException("New separator can't be `.` or `NULL`");
-        }
+        if (newSeparator.Length != 1)
+            throw new InvalidOperationException("new separator must be a single character.");
+        return ChangeSeparator(newSeparator[0]);
+    }
 
-        return newSeparator == "/"
-            ? CanonPath
-            : CanonPath.Replace("/", newSeparator);
+    /// <inheritdoc cref="ChangeSeparator(string)"/>
+    public string ChangeSeparator(char newSeparator)
+    {
+        if (newSeparator is '.' or '\0')
+            throw new ArgumentException("New separator can't be `.` or `NULL`");
+
+        // String.Replace() already checks if newSeparator == '/'
+        return CanonPath.Replace('/', newSeparator);
     }
 }
 

Robust.UnitTesting/Client/Sprite/SpriteBoundsTest.cs

@@ -0,0 +1,132 @@
+using System.Numerics;
+using System.Threading.Tasks;
+using NUnit.Framework;
+using Robust.Client;
+using Robust.Client.GameObjects;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Maths;
+
+namespace Robust.UnitTesting.Client.Sprite;
+
+public sealed class SpriteBoundsTest : RobustIntegrationTest
+{
+        private static readonly string Prototypes = @"
+- type: entity
+  id: spriteBoundsTest1
+  components:
+  - type: Sprite
+    sprite: debugRotation.rsi
+    layers:
+    - state: direction1
+
+- type: entity
+  id: spriteBoundsTest2
+  components:
+  - type: Sprite
+    sprite: debugRotation.rsi
+    layers:
+    - state: direction1
+    - state: direction1
+      rotation: 0.1
+
+- type: entity
+  id: spriteBoundsTest3
+  components:
+  - type: Sprite
+    sprite: debugRotation.rsi
+    layers:
+    - state: direction1
+    - state: direction1
+      rotation: 0.1
+      visible: false
+";
+
+    [Test]
+    public async Task TestSpriteBounds()
+    {
+        var client = StartClient(new ClientIntegrationOptions {ExtraPrototypes = Prototypes});
+        await client.WaitIdleAsync();
+        var baseClient = client.Resolve<IBaseClient>();
+
+        await client.WaitPost(() => baseClient.StartSinglePlayer());
+        await client.WaitIdleAsync();
+
+        var entMan = client.EntMan;
+        var sys = client.System<SpriteSystem>();
+
+        EntityUid uid1 = default;
+        EntityUid uid2 = default;
+        EntityUid uid3 = default;
+
+        await client.WaitPost(() =>
+        {
+            uid1 = entMan.Spawn("spriteBoundsTest1");
+            uid2 = entMan.Spawn("spriteBoundsTest2");
+            uid3 = entMan.Spawn("spriteBoundsTest3");
+        });
+
+        var ent1 = new Entity<SpriteComponent>(uid1, entMan.GetComponent<SpriteComponent>(uid1));
+        var ent2 = new Entity<SpriteComponent>(uid2, entMan.GetComponent<SpriteComponent>(uid2));
+        var ent3 = new Entity<SpriteComponent>(uid3, entMan.GetComponent<SpriteComponent>(uid3));
+
+        // None of the entities have empty bounding boxes
+        var box1 = sys.GetLocalBounds(ent1);
+        var box2 = sys.GetLocalBounds(ent2);
+        var box3 = sys.GetLocalBounds(ent3);
+        Assert.That(!box1.EqualsApprox(Box2.Empty));
+        Assert.That(!box2.EqualsApprox(Box2.Empty));
+        Assert.That(!box3.EqualsApprox(Box2.Empty));
+
+        // ent2 should have a larger bb than ent1 as it has a visible rotated layer
+        // ents 1 & 3 should have the same bounds, as the rotated layer is invisible in ent3
+        Assert.That(box1.EqualsApprox(box3));
+        Assert.That(!box1.EqualsApprox(box2));
+        Assert.That(box2.EqualsApprox(ent2.Comp.Layers[1].Bounds));
+        Assert.That(box2.Size.X, Is.GreaterThan(box1.Size.X));
+        Assert.That(box2.Size.Y, Is.GreaterThan(box1.Size.Y));
+
+        // Toggling layer visibility updates the bounds
+        sys.LayerSetVisible(ent2!, 1, false);
+        sys.LayerSetVisible(ent3!, 1, true);
+
+        var newBox2 = sys.GetLocalBounds(ent2);
+        var newBox3 = sys.GetLocalBounds(ent3);
+        Assert.That(!newBox2.EqualsApprox(Box2.Empty));
+        Assert.That(!newBox3.EqualsApprox(Box2.Empty));
+
+        Assert.That(box1.EqualsApprox(newBox2));
+        Assert.That(!box1.EqualsApprox(newBox3));
+        Assert.That(newBox3.EqualsApprox(ent3.Comp.Layers[1].Bounds));
+        Assert.That(newBox3.Size.X, Is.GreaterThan(box1.Size.X));
+        Assert.That(newBox3.Size.Y, Is.GreaterThan(box1.Size.Y));
+
+        // Changing the rotation, offset, scale, all trigger a bounds updatge
+        sys.LayerSetRotation(ent3!, 1, Angle.Zero);
+        var box = sys.GetLocalBounds(ent3);
+        Assert.That(box1.EqualsApprox(box));
+
+        // scale
+        sys.LayerSetScale(ent3!, 1, Vector2.One * 2);
+        box = sys.GetLocalBounds(ent3);
+        Assert.That(box1.EqualsApprox(box.Scale(0.5f)));
+        sys.LayerSetScale(ent3!, 1, Vector2.One);
+        box = sys.GetLocalBounds(ent3);
+        Assert.That(box1.EqualsApprox(box));
+
+        // offset
+        Assert.That(box.Center, Is.Approximately(Vector2.Zero));
+        sys.LayerSetOffset(ent3!, 1, Vector2.One);
+        box = sys.GetLocalBounds(ent3);
+        Assert.That(box.Size.X, Is.GreaterThan(box1.Size.X));
+        Assert.That(box.Size.Y, Is.GreaterThan(box1.Size.Y));
+        Assert.That(box.Center.X, Is.GreaterThan(0));
+        Assert.That(box.Center.Y, Is.GreaterThan(0));
+
+        await client.WaitPost(() =>
+        {
+            entMan.DeleteEntity(uid1);
+            entMan.DeleteEntity(uid2);
+            entMan.DeleteEntity(uid3);
+        });
+    }
+}

Robust.UnitTesting/IIntegrationInstance.cs

@@ -0,0 +1,118 @@
+using System;
+using System.Diagnostics.Contracts;
+using System.Threading;
+using System.Threading.Tasks;
+using Robust.Shared.Configuration;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Log;
+using Robust.Shared.Map;
+using Robust.Shared.Network;
+using Robust.Shared.Player;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Timing;
+
+namespace Robust.UnitTesting;
+
+public interface IIntegrationInstance : IDisposable
+{
+    /// <summary>
+    ///     Whether the instance is still alive.
+    ///     "Alive" indicates that it is able to receive and process commands.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">
+    ///     Thrown if you did not ensure that the instance is idle via <see cref="WaitIdleAsync"/> first.
+    /// </exception>
+    bool IsAlive { get; }
+
+    Exception? UnhandledException { get; }
+
+    EntityManager EntMan { get; }
+    IPrototypeManager ProtoMan { get; }
+    IConfigurationManager CfgMan { get; }
+    ISharedPlayerManager PlayerMan { get; }
+    INetManager NetMan { get; }
+    IMapManager MapMan { get; }
+    IGameTiming Timing { get; }
+    ISawmill Log { get; }
+
+    /// <summary>
+    ///     Resolve a dependency inside the instance.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">
+    ///     Thrown if you did not ensure that the instance is idle via <see cref="WaitIdleAsync"/> first.
+    /// </exception>
+    [Pure] T Resolve<T>();
+
+    [Pure] T System<T>() where T : IEntitySystem;
+
+    TransformComponent Transform(EntityUid uid);
+    MetaDataComponent MetaData(EntityUid uid);
+    MetaDataComponent MetaData(NetEntity uid);
+    TransformComponent Transform(NetEntity uid);
+
+    Task ExecuteCommand(string cmd);
+
+    /// <summary>
+    ///     Wait for the instance to go idle, either through finishing all commands or shutting down/crashing.
+    /// </summary>
+    /// <param name="throwOnUnhandled">
+    ///     If true, throw an exception if the server dies on an unhandled exception.
+    /// </param>
+    /// <param name="cancellationToken"></param>
+    /// <exception cref="Exception">
+    ///     Thrown if <paramref name="throwOnUnhandled"/> is true and the instance shuts down on an unhandled exception.
+    /// </exception>
+    Task WaitIdleAsync(bool throwOnUnhandled = true, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    ///     Queue for the server to run n ticks.
+    /// </summary>
+    /// <param name="ticks">The amount of ticks to run.</param>
+    void RunTicks(int ticks);
+
+    /// <summary>
+    ///     <see cref="RunTicks"/> followed by <see cref="WaitIdleAsync"/>
+    /// </summary>
+    Task WaitRunTicks(int ticks);
+
+    /// <summary>
+    ///     Queue for a delegate to be ran inside the main loop of the instance.
+    /// </summary>
+    /// <remarks>
+    ///     Do not run NUnit assertions inside <see cref="Post"/>. Use <see cref="Assert"/> instead.
+    /// </remarks>
+    void Post(Action post);
+
+    /// <inheritdoc cref="Post"/>
+    Task WaitPost(Action post);
+
+    /// <summary>
+    ///     Queue for a delegate to be ran inside the main loop of the instance,
+    ///     rethrowing any exceptions in <see cref="WaitIdleAsync"/>.
+    /// </summary>
+    /// <remarks>
+    ///     Exceptions raised inside this callback will be rethrown by <see cref="WaitIdleAsync"/>.
+    ///     This makes it ideal for NUnit assertions,
+    ///     since rethrowing the NUnit assertion directly provides less noise.
+    /// </remarks>
+    void Assert(Action assertion);
+
+    /// <inheritdoc cref="Assert"/>
+    Task WaitAssertion(Action assertion);
+
+    /// <summary>
+    /// Post-test cleanup
+    /// </summary>
+    Task Cleanup();
+}
+
+public interface IClientIntegrationInstance : IIntegrationInstance
+{
+    IClientNetManager CNetMan => (IClientNetManager) NetMan;
+    ICommonSession? Session { get; }
+    NetUserId? User { get; }
+    EntityUid? AttachedEntity { get; }
+    Task Connect(IServerIntegrationInstance target);
+}
+
+public interface IServerIntegrationInstance : IIntegrationInstance;

Robust.UnitTesting/Pool/ITestPair.cs

@@ -0,0 +1,39 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using Robust.Shared.Timing;
+
+namespace Robust.UnitTesting.Pool;
+
+public interface ITestPair
+{
+    int Id { get; }
+    public Stopwatch Watch { get; }
+    public PairState State { get; }
+    public bool Initialized { get; }
+    void Kill();
+    List<string> TestHistory { get; }
+    PairSettings Settings { get; set; }
+
+    int ServerSeed { get; }
+    int ClientSeed { get; }
+
+    void ActivateContext(TextWriter testOut);
+    void ValidateSettings(PairSettings settings);
+    void SetupSeed();
+    void ClearModifiedCvars();
+    void Use();
+    Task Init(int id, BasePoolManager manager, PairSettings settings, TextWriter testOut);
+    Task RecycleInternal(PairSettings next, TextWriter testOut);
+    Task ApplySettings(PairSettings settings);
+    Task RunTicksSync(int ticks);
+    Task SyncTicks(int targetDelta = 1);
+}
+
+public enum PairState : byte
+{
+    Ready = 0,
+    InUse = 1,
+    CleanDisposed = 2,
+    Dead = 3,
+}

Robust.UnitTesting/Pool/PairSettings.cs

@@ -0,0 +1,95 @@
+using System;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Random;
+
+namespace Robust.UnitTesting.Pool;
+
+/// <summary>
+/// Settings for a server-client pair. These settings may change over a pair's lifetime.
+/// The pool manager handles fetching pairs with a given setting, including applying new settings to re-used pairs.
+/// </summary>
+[Virtual]
+public class PairSettings
+{
+    /// <summary>
+    /// Set to true if the test will ruin the server/client pair.
+    /// </summary>
+    public virtual bool Destructive { get; init; }
+
+    /// <summary>
+    /// Set to true if the given server/client pair should be created fresh.
+    /// </summary>
+    public virtual bool Fresh { get; init; }
+
+    /// <summary>
+    /// Set to true if the given server/client pair should be connected from each other.
+    /// Defaults to disconnected as it makes dirty recycling slightly faster.
+    /// </summary>
+    public virtual bool Connected { get; init; }
+
+    /// <summary>
+    /// This will return a server-client pair that has not loaded test prototypes.
+    /// Try avoiding this whenever possible, as this will always  create & destroy a new pair.
+    /// Use <see cref="RobustTestPair.IsTestPrototype(EntityPrototype)"/> if you need to
+    /// exclude test prototypes.
+    /// </summary>
+    public virtual bool NoLoadTestPrototypes { get; init; }
+
+    /// <summary>
+    /// Set this to true to disable the NetInterp CVar on the given server/client pair
+    /// </summary>
+    public virtual bool DisableInterpolate { get; init; }
+
+    /// <summary>
+    /// Set this to true to always clean up the server/client pair before giving it to another borrower
+    /// </summary>
+    public virtual bool Dirty { get; init; }
+
+    /// <summary>
+    /// Overrides the test name detection, and uses this in the test history instead
+    /// </summary>
+    public virtual string? TestName { get; set; }
+
+    /// <summary>
+    /// If set, this will be used to call <see cref="IRobustRandom.SetSeed"/>
+    /// </summary>
+    public virtual int? ServerSeed { get; set; }
+
+    /// <summary>
+    /// If set, this will be used to call <see cref="IRobustRandom.SetSeed"/>
+    /// </summary>
+    public virtual int? ClientSeed { get; set; }
+
+    #region Inferred Properties
+
+    /// <summary>
+    /// If the returned pair must not be reused
+    /// </summary>
+    public virtual bool MustNotBeReused => Destructive || NoLoadTestPrototypes;
+
+    /// <summary>
+    /// If the given pair must be brand new
+    /// </summary>
+    public virtual bool MustBeNew => Fresh || NoLoadTestPrototypes;
+
+    #endregion
+
+    /// <summary>
+    /// Tries to guess if we can skip recycling the server/client pair.
+    /// </summary>
+    /// <param name="nextSettings">The next set of settings the old pair will be set to</param>
+    /// <returns>If we can skip cleaning it up</returns>
+    public virtual bool CanFastRecycle(PairSettings nextSettings)
+    {
+        if (MustNotBeReused)
+            throw new InvalidOperationException("Attempting to recycle a non-reusable test.");
+
+        if (nextSettings.MustBeNew)
+            throw new InvalidOperationException("Attempting to recycle a test while requesting a fresh test.");
+
+        if (Dirty)
+            return false;
+
+        return Connected == nextSettings.Connected;
+    }
+}

Robust.UnitTesting/Pool/PoolManager.cs

@@ -0,0 +1,332 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Reflection;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using NUnit.Framework;
+using Robust.Shared;
+using Robust.Shared.Timing;
+using Robust.Shared.Utility;
+
+namespace Robust.UnitTesting.Pool;
+
+public abstract class BasePoolManager
+{
+    internal abstract void Return(ITestPair pair);
+    public abstract Assembly[] ClientAssemblies { get; }
+    public abstract Assembly[] ServerAssemblies { get; }
+    public readonly List<string> TestPrototypes = new();
+
+    // default cvar overrides to use when creating test pairs.
+    public readonly List<(string cvar, string value)> DefaultCvars =
+    [
+        (CVars.NetPVS.Name, "false"),
+        (CVars.ThreadParallelCount.Name, "1"),
+        (CVars.ReplayClientRecordingEnabled.Name, "false"),
+        (CVars.ReplayServerRecordingEnabled.Name, "false"),
+        (CVars.NetBufferSize.Name, "0")
+    ];
+}
+
+[Virtual]
+public class PoolManager<TPair> : BasePoolManager where TPair : class, ITestPair, new()
+{
+    private int _nextPairId;
+    private readonly Lock _pairLock = new();
+    private bool _initialized;
+
+    /// <summary>
+    /// Set of all pairs, and whether they are currently in-use
+    /// </summary>
+    protected readonly Dictionary<TPair, bool> Pairs = new();
+    private bool _dead;
+    private Exception? _poolFailureReason;
+
+    private Assembly[] _clientAssemblies = [];
+    private Assembly[] _serverAssemblies = [];
+
+    public override Assembly[] ClientAssemblies => _clientAssemblies;
+    public override Assembly[] ServerAssemblies => _serverAssemblies;
+
+    /// <summary>
+    /// Initialize the pool manager. Override this to configure what assemblies should get loaded.
+    /// </summary>
+    public virtual void Startup(params Assembly[] extraAssemblies)
+    {
+        // By default, load no content assemblies, but make both server & client load the testing assembly.
+        Startup([], [], extraAssemblies);
+    }
+
+    protected void Startup(Assembly[] clientAssemblies, Assembly[] serverAssemblies, Assembly[] sharedAssemblies)
+    {
+        if (_initialized)
+            throw new InvalidOperationException("Already initialized");
+
+        DebugTools.AssertEqual(clientAssemblies.Intersect(sharedAssemblies).Count(), 0);
+        DebugTools.AssertEqual(serverAssemblies.Intersect(sharedAssemblies).Count(), 0);
+        DebugTools.AssertEqual(serverAssemblies.Intersect(clientAssemblies).Count(), 0);
+
+        foreach (var assembly in sharedAssemblies)
+        {
+            DiscoverTestPrototypes(assembly);
+        }
+
+        foreach (var assembly in clientAssemblies)
+        {
+            DiscoverTestPrototypes(assembly);
+        }
+
+        foreach (var assembly in serverAssemblies)
+        {
+            DiscoverTestPrototypes(assembly);
+        }
+
+        _initialized = true;
+        _clientAssemblies = clientAssemblies.Concat(sharedAssemblies).ToArray();
+        _serverAssemblies = serverAssemblies.Concat(sharedAssemblies).ToArray();
+    }
+
+    /// <summary>
+    /// This shuts down the pool, and disposes all the server/client pairs.
+    /// This is a one time operation to be used when the testing program is exiting.
+    /// </summary>
+    public void Shutdown()
+    {
+        List<TPair> localPairs;
+        lock (_pairLock)
+        {
+            if (_dead)
+                return;
+            _dead = true;
+            localPairs = Pairs.Keys.ToList();
+        }
+
+        foreach (var pair in localPairs)
+        {
+            pair.Kill();
+        }
+
+        _initialized = false;
+        TestPrototypes.Clear();
+    }
+
+    protected virtual string GetDefaultTestName(TestContext testContext)
+    {
+        return testContext.Test.FullName.Replace("Robust.UnitTesting.", "");
+    }
+
+    public string DeathReport()
+    {
+        lock (_pairLock)
+        {
+            var builder = new StringBuilder();
+            var pairs = Pairs.Keys.OrderBy(pair => pair.Id);
+            foreach (var pair in pairs)
+            {
+                var borrowed = Pairs[pair];
+                builder.AppendLine($"Pair {pair.Id}, Tests Run: {pair.TestHistory.Count}, Borrowed: {borrowed}");
+                for (var i = 0; i < pair.TestHistory.Count; i++)
+                {
+                    builder.AppendLine($"#{i}: {pair.TestHistory[i]}");
+                }
+            }
+
+            return builder.ToString();
+        }
+    }
+
+    public virtual PairSettings DefaultSettings => new();
+
+    public async Task<TPair> GetPair(PairSettings? settings = null)
+    {
+        if (!_initialized)
+            throw new InvalidOperationException($"Pool manager has not been initialized");
+
+        settings ??= DefaultSettings;
+
+        // Trust issues with the AsyncLocal that backs this.
+        var testContext = TestContext.CurrentContext;
+        var testOut = TestContext.Out;
+
+        DieIfPoolFailure();
+        var currentTestName = settings.TestName ?? GetDefaultTestName(testContext);
+        var watch = new Stopwatch();
+        await testOut.WriteLineAsync($"{nameof(GetPair)}: Called by test {currentTestName}");
+        TPair? pair = null;
+        try
+        {
+            watch.Start();
+            if (settings.MustBeNew)
+            {
+                await testOut.WriteLineAsync(
+                    $"{nameof(GetPair)}: Creating pair, because settings of pool settings");
+                pair = await CreateServerClientPair(settings, testOut);
+            }
+            else
+            {
+                await testOut.WriteLineAsync($"{nameof(GetPair)}: Looking in pool for a suitable pair");
+                pair = GrabOptimalPair(settings);
+                if (pair != null)
+                {
+                    pair.ActivateContext(testOut);
+                    await testOut.WriteLineAsync($"{nameof(GetPair)}: Suitable pair found");
+
+                    if (pair.Settings.CanFastRecycle(settings))
+                    {
+                        await testOut.WriteLineAsync($"{nameof(GetPair)}: Cleanup not needed, Skipping cleanup of pair");
+                        await pair.ApplySettings(settings);
+                    }
+                    else
+                    {
+                        await testOut.WriteLineAsync($"{nameof(GetPair)}: Cleaning existing pair");
+                        await pair.RecycleInternal(settings, testOut);
+                    }
+
+                    await pair.RunTicksSync(5);
+                    await pair.SyncTicks(targetDelta: 1);
+                }
+                else
+                {
+                    await testOut.WriteLineAsync($"{nameof(GetPair)}: Creating a new pair, no suitable pair found in pool");
+                    pair = await CreateServerClientPair(settings, testOut);
+                }
+            }
+        }
+        finally
+        {
+            if (pair != null && pair.TestHistory.Count > 0)
+            {
+                await testOut.WriteLineAsync($"{nameof(GetPair)}: Pair {pair.Id} Test History Start");
+                for (var i = 0; i < pair.TestHistory.Count; i++)
+                {
+                    await testOut.WriteLineAsync($"- Pair {pair.Id} Test #{i}: {pair.TestHistory[i]}");
+                }
+                await testOut.WriteLineAsync($"{nameof(GetPair)}: Pair {pair.Id} Test History End");
+            }
+        }
+
+        await testOut.WriteLineAsync($"{nameof(GetPair)}: Retrieving pair {pair.Id} from pool took {watch.Elapsed.TotalMilliseconds} ms");
+
+        pair.ValidateSettings(settings);
+        pair.ClearModifiedCvars();
+        pair.Settings = settings;
+        pair.TestHistory.Add(currentTestName);
+        pair.SetupSeed();
+
+        await testOut.WriteLineAsync($"{nameof(GetPair)}: Returning pair {pair.Id} with client/server seeds: {pair.ClientSeed}/{pair.ServerSeed}");
+
+        pair.Watch.Restart();
+        return pair;
+    }
+
+    private TPair? GrabOptimalPair(PairSettings poolSettings)
+    {
+        lock (_pairLock)
+        {
+            TPair? fallback = null;
+            foreach (var pair in Pairs.Keys)
+            {
+                if (Pairs[pair])
+                    continue;
+
+                if (!pair.Settings.CanFastRecycle(poolSettings))
+                {
+                    fallback = pair;
+                    continue;
+                }
+
+                pair.Use();
+                Pairs[pair] = true;
+                return pair;
+            }
+
+            if (fallback == null)
+                return null;
+
+            fallback.Use();
+            Pairs[fallback!] = true;
+            return fallback;
+        }
+    }
+
+    /// <summary>
+    /// Used by TestPair after checking the server/client pair, Don't use this.
+    /// </summary>
+    internal override void Return(ITestPair pair)
+    {
+        lock (_pairLock)
+        {
+            if (pair.State == PairState.Dead)
+                Pairs.Remove((TPair)pair);
+            else if (pair.State == PairState.Ready)
+                Pairs[(TPair) pair] = false;
+            else
+                throw new InvalidOperationException($"Attempted to return a pair in an invalid state. Pair: {pair.Id}. State: {pair.State}.");
+        }
+    }
+
+    private void DieIfPoolFailure()
+    {
+        if (_poolFailureReason != null)
+        {
+            // If the _poolFailureReason is not null, we can assume at least one test failed.
+            // So we say inconclusive so we don't add more failed tests to search through.
+            Assert.Inconclusive(@$"
+In a different test, the pool manager had an exception when trying to create a server/client pair.
+Instead of risking that the pool manager will fail at creating a server/client pairs for every single test,
+we are just going to end this here to save a lot of time. This is the exception that started this:\n {_poolFailureReason}");
+        }
+
+        if (_dead)
+        {
+            // If Pairs is null, we ran out of time, we can't assume a test failed.
+            // So we are going to tell it all future tests are a failure.
+            Assert.Fail("The pool was shut down");
+        }
+    }
+
+    private async Task<TPair> CreateServerClientPair(PairSettings settings, TextWriter testOut)
+    {
+        try
+        {
+            var id = Interlocked.Increment(ref _nextPairId);
+            var pair = new TPair();
+            await pair.Init(id, this, settings, testOut);
+            pair.Use();
+            await pair.RunTicksSync(5);
+            await pair.SyncTicks(targetDelta: 1);
+            return pair;
+        }
+        catch (Exception ex)
+        {
+            _poolFailureReason = ex;
+            throw;
+        }
+    }
+
+    private void DiscoverTestPrototypes(Assembly assembly)
+    {
+        const BindingFlags flags = BindingFlags.Static
+                                   | BindingFlags.NonPublic
+                                   | BindingFlags.Public
+                                   | BindingFlags.DeclaredOnly;
+
+        foreach (var type in assembly.GetTypes())
+        {
+            foreach (var field in type.GetFields(flags))
+            {
+                if (!field.HasCustomAttribute<TestPrototypesAttribute>())
+                    continue;
+
+                var val = field.GetValue(null);
+                if (val is not string str)
+                    throw new Exception($"{nameof(TestPrototypesAttribute)} is only valid on non-null string fields");
+
+                TestPrototypes.Add(str);
+            }
+        }
+    }
+}

Robust.UnitTesting/Pool/PoolTestLogHandler.cs

@@ -0,0 +1,77 @@
+using System;
+using System.IO;
+using NUnit.Framework;
+using Robust.Shared.Log;
+using Robust.Shared.Timing;
+using Serilog.Events;
+
+namespace Robust.UnitTesting.Pool;
+
+/// <summary>
+/// Log handler intended for pooled integration tests.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This class logs to two places: an NUnit <see cref="TestContext"/>  (so it nicely gets attributed to a test in your IDE),
+/// and an in-memory ring buffer for diagnostic purposes. If test pooling breaks, the ring buffer can be used to see what the broken instance has gone through.
+/// </para>
+/// <para>
+/// The active test context can be swapped out so pooled instances can correctly have their logs attributed.
+/// </para>
+/// </remarks>
+public sealed class PoolTestLogHandler : ILogHandler
+{
+    private readonly string? _prefix;
+
+    private RStopwatch _stopwatch;
+
+    public TextWriter? ActiveContext { get; private set; }
+
+    public LogLevel? FailureLevel { get; set; }
+
+    public PoolTestLogHandler(string? prefix)
+    {
+        _prefix = prefix != null ? $"{prefix}: " : "";
+    }
+
+    public bool ShuttingDown;
+
+    public void Log(string sawmillName, LogEvent message)
+    {
+        var level = message.Level.ToRobust();
+
+        if (ShuttingDown && (FailureLevel == null || level < FailureLevel))
+            return;
+
+        if (ActiveContext is not { } testContext)
+        {
+            // If this gets hit it means something is logging to this instance while it's "between" tests.
+            // This is a bug in either the game or the testing system, and must always be investigated.
+            throw new InvalidOperationException("Log to pool test log handler without active test context");
+        }
+
+        var name = LogMessage.LogLevelToName(level);
+        var seconds = _stopwatch.Elapsed.TotalSeconds;
+        var rendered = message.RenderMessage();
+        var line = $"{_prefix}{seconds:F3}s [{name}] {sawmillName}: {rendered}";
+
+        testContext.WriteLine(line);
+
+        if (FailureLevel == null || level < FailureLevel)
+            return;
+
+        testContext.Flush();
+        Assert.Fail($"{line} Exception: {message.Exception}");
+    }
+
+    public void ClearContext()
+    {
+        ActiveContext = null;
+    }
+
+    public void ActivateContext(TextWriter context)
+    {
+        _stopwatch.Restart();
+        ActiveContext = context;
+    }
+}

Robust.UnitTesting/Pool/TestMapData.cs

@@ -0,0 +1,23 @@
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map;
+using Robust.Shared.Map.Components;
+
+namespace Robust.UnitTesting.Pool;
+
+/// <summary>
+/// Simple data class that stored information about a map being used by a test.
+/// </summary>
+public sealed class TestMapData
+{
+    public EntityUid MapUid { get; set; }
+    public Entity<MapGridComponent> Grid;
+    public MapId MapId;
+    public EntityCoordinates GridCoords { get; set; }
+    public MapCoordinates MapCoords { get; set; }
+    public TileRef Tile { get; set; }
+
+    // Client-side uids
+    public EntityUid CMapUid { get; set; }
+    public EntityUid CGridUid { get; set; }
+    public EntityCoordinates CGridCoords { get; set; }
+}

Robust.UnitTesting/Pool/TestPair.Helpers.cs

@@ -0,0 +1,285 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Threading.Tasks;
+using NUnit.Framework;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map;
+using Robust.Shared.Prototypes;
+
+namespace Robust.UnitTesting.Pool;
+
+// This partial file contains misc helper functions to make writing tests easier.
+public partial class TestPair<TServer, TClient>
+{
+    /// <summary>
+    /// Convert a client-side uid into a server-side uid
+    /// </summary>
+    public EntityUid ToServerUid(EntityUid uid) => ConvertUid(uid, Client, Server);
+
+    /// <summary>
+    /// Convert a server-side uid into a client-side uid
+    /// </summary>
+    public EntityUid ToClientUid(EntityUid uid) => ConvertUid(uid, Server, Client);
+
+    private static EntityUid ConvertUid(EntityUid uid, IIntegrationInstance source, IIntegrationInstance destination)
+    {
+        if (!uid.IsValid())
+            return EntityUid.Invalid;
+
+        if (!source.EntMan.TryGetComponent<MetaDataComponent>(uid, out var meta))
+        {
+            Assert.Fail($"Failed to resolve MetaData while converting the EntityUid for entity {uid}");
+            return EntityUid.Invalid;
+        }
+
+        if (!destination.EntMan.TryGetEntity(meta.NetEntity, out var otherUid))
+        {
+            Assert.Fail($"Failed to resolve net ID while converting the EntityUid entity {source.EntMan.ToPrettyString(uid)}");
+            return EntityUid.Invalid;
+        }
+
+        return otherUid.Value;
+    }
+
+    /// <summary>
+    /// Execute a command on the server and wait some number of ticks.
+    /// </summary>
+    public async Task WaitCommand(string cmd, int numTicks = 10)
+    {
+        await Server.ExecuteCommand(cmd);
+        await RunTicksSync(numTicks);
+    }
+
+    /// <summary>
+    /// Execute a command on the client and wait some number of ticks.
+    /// </summary>
+    public async Task WaitClientCommand(string cmd, int numTicks = 10)
+    {
+        await Client.ExecuteCommand(cmd);
+        await RunTicksSync(numTicks);
+    }
+
+    /// <summary>
+    /// Retrieve all entity prototypes that have some component.
+    /// </summary>
+    public List<(EntityPrototype, T)> GetPrototypesWithComponent<T>(
+        HashSet<string>? ignored = null,
+        bool ignoreAbstract = true,
+        bool ignoreTestPrototypes = true)
+        where T : IComponent, new()
+    {
+        if (!Server.Resolve<IComponentFactory>().TryGetRegistration<T>(out var reg)
+            && !Client.Resolve<IComponentFactory>().TryGetRegistration<T>(out reg))
+        {
+            Assert.Fail($"Unknown component: {typeof(T).Name}");
+            return new();
+        }
+
+        var id = reg.Name;
+        var list = new List<(EntityPrototype, T)>();
+        foreach (var proto in Server.ProtoMan.EnumeratePrototypes<EntityPrototype>())
+        {
+            if (ignored != null && ignored.Contains(proto.ID))
+                continue;
+
+            if (ignoreAbstract && proto.Abstract)
+                continue;
+
+            if (ignoreTestPrototypes && IsTestPrototype(proto))
+                continue;
+
+            if (proto.Components.TryGetComponent(id, out var cmp))
+                list.Add((proto, (T)cmp));
+        }
+
+        return list;
+    }
+
+    /// <summary>
+    /// Retrieve all entity prototypes that have some component.
+    /// </summary>
+    public List<EntityPrototype> GetPrototypesWithComponent(
+        Type type,
+        HashSet<string>? ignored = null,
+        bool ignoreAbstract = true,
+        bool ignoreTestPrototypes = true)
+    {
+        if (!Server.Resolve<IComponentFactory>().TryGetRegistration(type, out var reg)
+            && !Client.Resolve<IComponentFactory>().TryGetRegistration(type, out reg))
+        {
+            Assert.Fail($"Unknown component: {type.Name}");
+            return new();
+        }
+
+        var id = reg.Name;
+        var list = new List<EntityPrototype>();
+        foreach (var proto in Server.ProtoMan.EnumeratePrototypes<EntityPrototype>())
+        {
+            if (ignored != null && ignored.Contains(proto.ID))
+                continue;
+
+            if (ignoreAbstract && proto.Abstract)
+                continue;
+
+            if (ignoreTestPrototypes && IsTestPrototype(proto))
+                continue;
+
+            if (proto.Components.ContainsKey(id))
+                list.Add((proto));
+        }
+
+        return list;
+    }
+
+    public async Task Connect()
+    {
+        Assert.That(Client.NetMan.IsConnected, Is.False);
+        await Client.Connect(Server);
+        await ReallyBeIdle(10);
+        await Client.WaitRunTicks(1);
+    }
+
+    public async Task Disconnect(string reason = "")
+    {
+        await Client.WaitPost(() => Client.CNetMan.ClientDisconnect(reason));
+        await ReallyBeIdle(10);
+    }
+
+    public bool IsTestPrototype(EntityPrototype proto)
+    {
+        return _loadedEntityPrototypes.Contains(proto.ID);
+    }
+
+    public bool IsTestEntityPrototype(string id)
+    {
+        return _loadedEntityPrototypes.Contains(id);
+    }
+
+    public bool IsTestPrototype<TPrototype>(string id) where TPrototype : IPrototype
+    {
+        return IsTestPrototype(typeof(TPrototype), id);
+    }
+
+    public bool IsTestPrototype<TPrototype>(TPrototype proto) where TPrototype : IPrototype
+    {
+        return IsTestPrototype(typeof(TPrototype), proto.ID);
+    }
+
+    public bool IsTestPrototype(Type kind, string id)
+    {
+        return _loadedPrototypes.TryGetValue(kind, out var ids) && ids.Contains(id);
+    }
+
+    /// <summary>
+    /// Runs the server-client pair in sync
+    /// </summary>
+    /// <param name="ticks">How many ticks to run them for</param>
+    public async Task RunTicksSync(int ticks)
+    {
+        for (var i = 0; i < ticks; i++)
+        {
+            await Server.WaitRunTicks(1);
+            await Client.WaitRunTicks(1);
+        }
+    }
+
+    /// <summary>
+    /// Convert a time interval to some number of ticks.
+    /// </summary>
+    public int SecondsToTicks(float seconds)
+    {
+        return (int) Math.Ceiling(seconds / Server.Timing.TickPeriod.TotalSeconds);
+    }
+
+    /// <summary>
+    /// Run the server & client in sync for some amount of time
+    /// </summary>
+    public async Task RunSeconds(float seconds)
+    {
+        await RunTicksSync(SecondsToTicks(seconds));
+    }
+
+    /// <summary>
+    /// Runs the server-client pair in sync, but also ensures they are both idle each tick.
+    /// </summary>
+    /// <param name="runTicks">How many ticks to run</param>
+    public async Task ReallyBeIdle(int runTicks = 25)
+    {
+        for (var i = 0; i < runTicks; i++)
+        {
+            await Client.WaitRunTicks(1);
+            await Server.WaitRunTicks(1);
+            for (var idleCycles = 0; idleCycles < 4; idleCycles++)
+            {
+                await Client.WaitIdleAsync();
+                await Server.WaitIdleAsync();
+            }
+        }
+    }
+
+    /// <summary>
+    /// Run the server/clients until the ticks are synchronized.
+    /// By default the client will be one tick ahead of the server.
+    /// </summary>
+    public async Task SyncTicks(int targetDelta = 1)
+    {
+        var sTick = (int)Server.Timing.CurTick.Value;
+        var cTick = (int)Client.Timing.CurTick.Value;
+        var delta = cTick - sTick;
+
+        if (delta == targetDelta)
+            return;
+        if (delta > targetDelta)
+            await Server.WaitRunTicks(delta - targetDelta);
+        else
+            await Client.WaitRunTicks(targetDelta - delta);
+
+        sTick = (int)Server.Timing.CurTick.Value;
+        cTick = (int)Client.Timing.CurTick.Value;
+        delta = cTick - sTick;
+        Assert.That(delta, Is.EqualTo(targetDelta));
+    }
+
+    /// <summary>
+    /// Creates a map with a single grid consisting of one tile.
+    /// </summary>
+    [MemberNotNull(nameof(TestMap))]
+    public async Task<TestMapData> CreateTestMap(bool initialized, ushort tileTypeId)
+    {
+        TestMap = new TestMapData();
+        await Server.WaitIdleAsync();
+        var sys = Server.System<SharedMapSystem>();
+
+        await Server.WaitPost(() =>
+        {
+            TestMap.MapUid = sys.CreateMap(out TestMap.MapId, runMapInit: initialized);
+            TestMap.Grid = Server.MapMan.CreateGridEntity(TestMap.MapId);
+            TestMap.GridCoords = new EntityCoordinates(TestMap.Grid, 0, 0);
+            TestMap.MapCoords = new MapCoordinates(0, 0, TestMap.MapId);
+            sys.SetTile(TestMap.Grid.Owner, TestMap.Grid.Comp, TestMap.GridCoords, new Tile(tileTypeId));
+            TestMap.Tile = sys.GetAllTiles(TestMap.Grid.Owner, TestMap.Grid.Comp).First();
+        });
+
+        if (!Settings.Connected)
+            return TestMap;
+
+        await RunTicksSync(10);
+        TestMap.CMapUid = ToClientUid(TestMap.MapUid);
+        TestMap.CGridUid = ToClientUid(TestMap.Grid);
+        TestMap.CGridCoords = new EntityCoordinates(TestMap.CGridUid, 0, 0);
+
+        return TestMap;
+    }
+
+    /// <inheritdoc cref="CreateTestMap(bool, ushort)"/>
+    [MemberNotNull(nameof(TestMap))]
+    public async Task<TestMapData> CreateTestMap(bool initialized, string tileName)
+    {
+        var defMan = Server.Resolve<ITileDefinitionManager>();
+        if (!defMan.TryGetDefinition(tileName, out var def))
+            Assert.Fail($"Unknown tile: {tileName}");
+        return await CreateTestMap(initialized, def?.TileId ?? 1);
+    }
+}

Robust.UnitTesting/Pool/TestPair.Recycle.cs

@@ -0,0 +1,208 @@
+using System;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using NUnit.Framework;
+using Robust.Client;
+using Robust.Shared;
+using Robust.Shared.Exceptions;
+using Robust.Shared.Network;
+using Robust.Shared.Player;
+
+namespace Robust.UnitTesting.Pool;
+
+// This partial file contains logic related to recycling & disposing test pairs.
+public partial class TestPair<TServer, TClient>
+{
+    private async Task OnDirtyDispose()
+    {
+        var usageTime = Watch.Elapsed;
+        Watch.Restart();
+        await TestOut.WriteLineAsync($"{nameof(DisposeAsync)}: Test gave back pair {Id} in {usageTime.TotalMilliseconds} ms");
+        Kill();
+        var disposeTime = Watch.Elapsed;
+        await TestOut.WriteLineAsync($"{nameof(DisposeAsync)}: Disposed pair {Id} in {disposeTime.TotalMilliseconds} ms");
+        // Test pairs should only dirty dispose if they are failing. If they are not failing, this probably happened
+        // because someone forgot to clean-return the pair.
+        Assert.Warn("Test was dirty-disposed.");
+    }
+
+    /// <summary>
+    /// This method gets called before any test pair gets returned to the pool.
+    /// </summary>
+    protected virtual async Task Cleanup()
+    {
+        if (TestMap != null)
+        {
+            await Server.WaitPost(() => Server.EntMan.DeleteEntity(TestMap.MapUid));
+            TestMap = null;
+        }
+    }
+
+    private async Task OnCleanDispose()
+    {
+        await Server.WaitIdleAsync();
+        await Client.WaitIdleAsync();
+        await Cleanup();
+        await Server.Cleanup();
+        await Client.Cleanup();
+        await RevertModifiedCvars();
+
+        var usageTime = Watch.Elapsed;
+        Watch.Restart();
+        await TestOut.WriteLineAsync($"{nameof(CleanReturnAsync)}: Test borrowed pair {Id} for {usageTime.TotalMilliseconds} ms");
+        // Let any last minute failures the test cause happen.
+        await ReallyBeIdle();
+        if (!Settings.Destructive)
+        {
+            if (Client.IsAlive == false)
+                throw new Exception($"{nameof(CleanReturnAsync)}: Test killed the client in pair {Id}:", Client.UnhandledException);
+
+            if (Server.IsAlive == false)
+                throw new Exception($"{nameof(CleanReturnAsync)}: Test killed the server in pair {Id}:", Server.UnhandledException);
+        }
+
+        if (Settings.MustNotBeReused)
+        {
+            Kill();
+            await ReallyBeIdle();
+            await TestOut.WriteLineAsync($"{nameof(CleanReturnAsync)}: Clean disposed in {Watch.Elapsed.TotalMilliseconds} ms");
+            return;
+        }
+
+        var sRuntimeLog = Server.Resolve<IRuntimeLog>();
+        if (sRuntimeLog.ExceptionCount > 0)
+            throw new Exception($"{nameof(CleanReturnAsync)}: Server logged exceptions");
+        var cRuntimeLog = Client.Resolve<IRuntimeLog>();
+        if (cRuntimeLog.ExceptionCount > 0)
+            throw new Exception($"{nameof(CleanReturnAsync)}: Client logged exceptions");
+
+        var returnTime = Watch.Elapsed;
+        await TestOut.WriteLineAsync($"{nameof(CleanReturnAsync)}: PoolManager took {returnTime.TotalMilliseconds} ms to put pair {Id} back into the pool");
+    }
+
+    public async ValueTask CleanReturnAsync()
+    {
+        if (State != PairState.InUse)
+            throw new Exception($"{nameof(CleanReturnAsync)}: Unexpected state. Pair: {Id}. State: {State}.");
+
+        await TestOut.WriteLineAsync($"{nameof(CleanReturnAsync)}: Return of pair {Id} started");
+        State = PairState.CleanDisposed;
+        await OnCleanDispose();
+        State = PairState.Ready;
+        Manager.Return(this);
+        ClearContext();
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        switch (State)
+        {
+            case PairState.Dead:
+            case PairState.Ready:
+                break;
+            case PairState.InUse:
+                await TestOut.WriteLineAsync($"{nameof(DisposeAsync)}: Dirty return of pair {Id} started");
+                await OnDirtyDispose();
+                Manager.Return(this);
+                ClearContext();
+                break;
+            default:
+                throw new Exception($"{nameof(DisposeAsync)}: Unexpected state. Pair: {Id}. State: {State}.");
+        }
+    }
+
+    /// <summary>
+    /// This method gets called when a previously used test pair is being retrieved from the pool.
+    /// Note that in some instances this method may get skipped (See <see cref="PairSettings.CanFastRecycle"/>).
+    /// </summary>
+    public async Task RecycleInternal(PairSettings settings, TextWriter testOut)
+    {
+        Watch.Restart();
+        await testOut.WriteLineAsync($"Recycling...");
+        await RunTicksSync(1);
+
+        // Disconnect the client if they are connected.
+        if (Client.CNetMan.IsConnected)
+        {
+            await testOut.WriteLineAsync($"Recycling: {Watch.Elapsed.TotalMilliseconds} ms: Disconnecting client.");
+            await Client.WaitPost(() => Client.CNetMan.ClientDisconnect("Test pooling cleanup disconnect"));
+            await RunTicksSync(1);
+        }
+
+        await Recycle(settings, testOut);
+        ClearModifiedCvars();
+
+        // (possibly) reconnect the client
+        if (settings.Connected)
+        {
+            await testOut.WriteLineAsync($"Recycling: {Watch.Elapsed.TotalMilliseconds} ms: Connecting client");
+            await Client.Connect(Server);
+        }
+
+        Settings = default!;
+        await testOut.WriteLineAsync($"Recycling: {Watch.Elapsed.TotalMilliseconds} ms: Idling");
+        await ReallyBeIdle();
+        await testOut.WriteLineAsync($"Recycling: {Watch.Elapsed.TotalMilliseconds} ms: Done recycling");
+    }
+
+    /// <summary>
+    /// This method gets called when a previously used test pair is being retrieved from the pool.
+    /// If the next settings are compatible with the previous settings, this step may get skipped (See <see cref="PairSettings.CanFastRecycle"/>).
+    /// In general, this method should also call <see cref="ApplySettings"/>.
+    /// </summary>
+    protected virtual async Task Recycle(PairSettings next, TextWriter testOut)
+    {
+        //Apply Cvars
+        await testOut.WriteLineAsync($"Recycling: {Watch.Elapsed.TotalMilliseconds} ms: Setting CVar ");
+        await ApplySettings(next);
+        await RunTicksSync(1);
+
+        // flush server entities.
+        await testOut.WriteLineAsync($"Recycling: {Watch.Elapsed.TotalMilliseconds} ms: Flushing server entities");
+        await Server.WaitPost(() => Server.EntMan.FlushEntities());
+        await RunTicksSync(1);
+    }
+
+    /// <summary>
+    /// Apply settings to the test pair. This method is always called when a pair is fetched from the pool. There should
+    /// be no need to apply settings that require a pair to be recycled, as in those cases the
+    /// <see cref="PairSettings.CanFastRecycle"/> should have caused <see cref="Recycle"/> to be invoked, which should
+    /// already have applied those settings.
+    /// </summary>
+    public async Task ApplySettings(PairSettings next)
+    {
+        await ApplySettings(Client, next);
+        await ApplySettings(Server, next);
+    }
+
+    /// <inheritdoc cref="ApplySettings(PairSettings)"/>
+    [MustCallBase]
+    protected internal virtual async Task ApplySettings(IIntegrationInstance instance, PairSettings next)
+    {
+        if (instance.CfgMan.IsCVarRegistered(CVars.NetInterp.Name))
+            await instance.WaitPost(() => instance.CfgMan.SetCVar(CVars.NetInterp, !next.DisableInterpolate));
+    }
+
+    /// <summary>
+    /// Invoked after a test pair has been recycled to validate that the settings have been properly applied.
+    /// </summary>
+    [MustCallBase]
+    public virtual void ValidateSettings(PairSettings settings)
+    {
+        var netMan = Client.Resolve<INetManager>();
+        Assert.That(netMan.IsConnected, Is.EqualTo(settings.Connected));
+
+        if (!settings.Connected)
+            return;
+
+        var baseClient = Client.Resolve<IBaseClient>();
+        var cPlayer = Client.Resolve<ISharedPlayerManager>();
+        var sPlayer = Server.Resolve<ISharedPlayerManager>();
+
+        Assert.That(baseClient.RunLevel, Is.EqualTo(ClientRunLevel.InGame));
+        Assert.That(sPlayer.Sessions.Length, Is.EqualTo(1));
+        var session = sPlayer.Sessions.Single();
+        Assert.That(cPlayer.LocalSession?.UserId, Is.EqualTo(session.UserId));
+    }
+}

Robust.UnitTesting/Pool/TestPair.cs

@@ -0,0 +1,244 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using Robust.Shared.Configuration;
+using Robust.Shared.Player;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Random;
+using Robust.Shared.Timing;
+using Robust.Shared.Utility;
+
+namespace Robust.UnitTesting.Pool;
+
+/// <summary>
+/// This object wraps a pooled server+client pair.
+/// </summary>
+public abstract partial class TestPair<TServer, TClient> : ITestPair, IAsyncDisposable
+    where TServer : IServerIntegrationInstance
+    where TClient : IClientIntegrationInstance
+{
+    public int Id { get; internal set; }
+    protected BasePoolManager Manager = default!;
+    public PairState State { get; private set; } = PairState.Ready;
+    public bool Initialized { get; private set; }
+    protected TextWriter TestOut = default!;
+    public Stopwatch Watch { get; } = new();
+    public List<string> TestHistory { get; } = new();
+    public PairSettings Settings { get; set; } = default!;
+
+    public readonly PoolTestLogHandler ServerLogHandler = new("SERVER");
+    public readonly PoolTestLogHandler ClientLogHandler = new("CLIENT");
+    public TestMapData? TestMap;
+
+    private int _nextServerSeed;
+    private int _nextClientSeed;
+
+    public int ServerSeed { get; set; }
+    public int ClientSeed { get; set; }
+
+    public TServer Server { get; private set; } = default!;
+    public TClient Client { get; private set; } = default!;
+
+    public ICommonSession? Player => Server.PlayerMan.SessionsDict.GetValueOrDefault(Client.User ?? default);
+
+    private Dictionary<Type, HashSet<string>> _loadedPrototypes = new();
+    private HashSet<string> _loadedEntityPrototypes = new();
+    protected readonly Dictionary<string, object> ModifiedClientCvars = new();
+    protected readonly Dictionary<string, object> ModifiedServerCvars = new();
+
+    public async Task LoadPrototypes(List<string> prototypes)
+    {
+        await LoadPrototypes(Server, prototypes);
+        await LoadPrototypes(Client, prototypes);
+    }
+
+    public async Task Init(
+        int id,
+        BasePoolManager manager,
+        PairSettings settings,
+        TextWriter testOut)
+    {
+        if (Initialized)
+            throw new InvalidOperationException("Already initialized");
+
+        Id = id;
+        Manager = manager;
+        Settings = settings;
+        Initialized = true;
+
+        ClientLogHandler.ActivateContext(testOut);
+        ServerLogHandler.ActivateContext(testOut);
+        Client = await GenerateClient();
+        Server = await GenerateServer();
+        ActivateContext(testOut);
+        await ApplySettings(settings);
+
+        Client.CfgMan.OnCVarValueChanged += OnClientCvarChanged;
+        Server.CfgMan.OnCVarValueChanged += OnServerCvarChanged;
+
+        if (!settings.NoLoadTestPrototypes)
+            await LoadPrototypes(Manager.TestPrototypes);
+
+        var cRand = Client.Resolve<IRobustRandom>();
+        var sRand = Server.Resolve<IRobustRandom>();
+        _nextClientSeed = cRand.Next();
+        _nextServerSeed = sRand.Next();
+
+        await Initialize();
+
+        // Always initially connect clients.
+        // This is done in case the server does randomization when client first connects
+        // This is to try and prevent issues where if the first test that connects the client is consistently some test
+        // that uses a fixed seed, it would effectively prevent the initial configuration from being randomized.
+        await Connect();
+
+        if (!Settings.Connected)
+            await Disconnect("Initial disconnect");
+    }
+
+    protected virtual Task Initialize()
+    {
+        return Task.CompletedTask;
+    }
+
+    protected abstract Task<TClient> GenerateClient();
+    protected abstract Task<TServer> GenerateServer();
+
+    public void Kill()
+    {
+        State = PairState.Dead;
+        ServerLogHandler.ShuttingDown = true;
+        ClientLogHandler.ShuttingDown = true;
+        Server.Dispose();
+        Client.Dispose();
+    }
+
+    private void ClearContext()
+    {
+        TestOut = default!;
+        ServerLogHandler.ClearContext();
+        ClientLogHandler.ClearContext();
+    }
+
+    public void ActivateContext(TextWriter testOut)
+    {
+        TestOut = testOut;
+        ServerLogHandler.ActivateContext(testOut);
+        ClientLogHandler.ActivateContext(testOut);
+    }
+
+    public void Use()
+    {
+        if (State != PairState.Ready)
+            throw new InvalidOperationException($"Pair is not ready to use. State: {State}");
+        State = PairState.InUse;
+    }
+
+    public void SetupSeed()
+    {
+        var sRand = Server.Resolve<IRobustRandom>();
+        if (Settings.ServerSeed is { } severSeed)
+        {
+            ServerSeed = severSeed;
+            sRand.SetSeed(ServerSeed);
+        }
+        else
+        {
+            ServerSeed = _nextServerSeed;
+            sRand.SetSeed(ServerSeed);
+            _nextServerSeed = sRand.Next();
+        }
+
+        var cRand = Client.Resolve<IRobustRandom>();
+        if (Settings.ClientSeed is { } clientSeed)
+        {
+            ClientSeed = clientSeed;
+            cRand.SetSeed(ClientSeed);
+        }
+        else
+        {
+            ClientSeed = _nextClientSeed;
+            cRand.SetSeed(ClientSeed);
+            _nextClientSeed = cRand.Next();
+        }
+    }
+
+    private async Task LoadPrototypes(IIntegrationInstance instance, List<string> prototypes)
+    {
+        var changed = new Dictionary<Type, HashSet<string>>();
+        foreach (var file in prototypes)
+        {
+            instance.ProtoMan.LoadString(file, changed: changed);
+        }
+
+        await instance.WaitPost(() => instance.ProtoMan.ReloadPrototypes(changed));
+
+        foreach (var (kind, ids) in changed)
+        {
+            _loadedPrototypes.GetOrNew(kind).UnionWith(ids);
+        }
+
+        if (_loadedPrototypes.TryGetValue(typeof(EntityPrototype), out var entIds))
+            _loadedEntityPrototypes.UnionWith(entIds);
+    }
+
+    public void Deconstruct(out TServer server, out TClient client)
+    {
+        server = Server;
+        client = Client;
+    }
+
+    private void OnServerCvarChanged(CVarChangeInfo args)
+    {
+        ModifiedServerCvars.TryAdd(args.Name, args.OldValue);
+    }
+
+    private void OnClientCvarChanged(CVarChangeInfo args)
+    {
+        ModifiedClientCvars.TryAdd(args.Name, args.OldValue);
+    }
+
+    public void ClearModifiedCvars()
+    {
+        ModifiedClientCvars.Clear();
+        ModifiedServerCvars.Clear();
+    }
+
+    /// <summary>
+    /// Reverts any cvars that were modified during a test back to their original values.
+    /// </summary>
+    public virtual async Task RevertModifiedCvars()
+    {
+        await Server.WaitPost(() =>
+        {
+            foreach (var (name, value) in ModifiedServerCvars)
+            {
+                if (Server.CfgMan.GetCVar(name).Equals(value))
+                    continue;
+
+                Server.Log.Info($"Resetting cvar {name} to {value}");
+                Server.CfgMan.SetCVar(name, value);
+            }
+
+        });
+
+        await Client.WaitPost(() =>
+        {
+            foreach (var (name, value) in ModifiedClientCvars)
+            {
+                if (Client.CfgMan.GetCVar(name).Equals(value))
+                    continue;
+
+                var flags = Client.CfgMan.GetCVarFlags(name);
+                if (flags.HasFlag(CVar.REPLICATED) && flags.HasFlag(CVar.SERVER))
+                    continue;
+
+                Client.Log.Info($"Resetting cvar {name} to {value}");
+                Client.CfgMan.SetCVar(name, value);
+            }
+        });
+
+        ClearModifiedCvars();
+    }
+}