Don't do work if assigning stylesheet a control already has

RELEASE-NOTES.md

@@ -51,6 +51,7 @@ END TEMPLATE-->
 ### Other
 
 * Add pure to some SharedTransformSystem methods.
+* `Control.Stylesheet` does not do any work if assigning the value it already has.
 
 ### Internal
 

Robust.Client/UserInterface/Control.Styling.cs

@@ -19,6 +19,9 @@ namespace Robust.Client.UserInterface
             get => _stylesheet;
             set
             {
+                if (ReferenceEquals(_stylesheet, value))
+                    return;
+
                 _stylesheet = value;
                 StylesheetUpdateRecursive();
             }