Fix OutputPanel.SetMessage causing the panel to bounce if setting a message other than the last one (#6163)

Robust.Client/UserInterface/Controls/OutputPanel.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.Contracts;
 using System.Numerics;
 using Robust.Client.Graphics;
 using Robust.Client.UserInterface.RichText;
@@ -154,6 +155,7 @@ namespace Robust.Client.UserInterface.Controls
 
         public void SetMessage(Index index, FormattedMessage message, Type[]? tagsAllowed = null, Color? defaultColor = null)
         {
+            var atBottom = !_scrollDownButton.Visible;
             var oldEntry = _entries[index];
             var font = _getFont();
             _totalContentHeight -= oldEntry.Height + font.GetLineSeparation(UIScale);
@@ -164,6 +166,10 @@ namespace Robust.Client.UserInterface.Controls
             _entries[index] = entry;
 
             AddNewItemHeight(font, in entry);
+
+            _scrollBar.MaxValue = Math.Max(_scrollBar.Page, _totalContentHeight);
+            if (atBottom)
+                _scrollBar.Value = _scrollBar.MaxValue;
         }
 
         private void AddNewItemHeight(Font font, in RichTextEntry entry)
@@ -278,7 +284,7 @@ namespace Robust.Client.UserInterface.Controls
             }
         }
 
-        [System.Diagnostics.Contracts.Pure]
+        [Pure]
         private Font _getFont()
         {
             if (TryGetStyleProperty<Font>("font", out var font))
@@ -289,7 +295,7 @@ namespace Robust.Client.UserInterface.Controls
             return UserInterfaceManager.ThemeDefaults.DefaultFont;
         }
 
-        [System.Diagnostics.Contracts.Pure]
+        [Pure]
         private StyleBox? _getStyleBox()
         {
             if (StyleBoxOverride != null)
@@ -301,14 +307,14 @@ namespace Robust.Client.UserInterface.Controls
             return box;
         }
 
-        [System.Diagnostics.Contracts.Pure]
+        [Pure]
         private float _getScrollSpeed()
         {
             // The scroll speed depends on the UI scale because the scroll bar is working with physical pixels.
             return GetScrollSpeed(_getFont(), UIScale);
         }
 
-        [System.Diagnostics.Contracts.Pure]
+        [Pure]
         private UIBox2 _getContentBox()
         {
             var style = _getStyleBox();