Sync BackgroundSizing in composition border visuals

src/Avalonia.Controls/Border.cs

@@ -81,6 +81,10 @@ namespace Avalonia.Controls
                 case nameof(BorderThickness):
                     _layoutThickness = null;
                     break;
+                case nameof(BackgroundSizing):
+                    if (_borderVisual != null)
+                        _borderVisual.BackgroundSizing = BackgroundSizing;
+                    break;
                 case nameof(CornerRadius):
                     if (_borderVisual != null)
                         _borderVisual.CornerRadius = CornerRadius;
@@ -215,6 +219,7 @@ namespace Avalonia.Controls
         {
             return _borderVisual = new CompositionBorderVisual(compositor, this)
             {
+                BackgroundSizing = BackgroundSizing,
                 CornerRadius = CornerRadius
             };
         }

src/Avalonia.Controls/BorderVisual.cs

@@ -15,6 +15,8 @@ class CompositionBorderVisual : CompositionDrawListVisual
     private bool _cornerRadiusChanged;
     private Thickness _borderThickness;
     private bool _borderThicknessChanged;
+    private BackgroundSizing _backgroundSizing;
+    private bool _backgroundSizingChanged;
     
     public CompositionBorderVisual(Compositor compositor, Visual visual) : base(compositor,
         new ServerBorderVisual(compositor.Server, visual), visual)
@@ -49,6 +51,20 @@ class CompositionBorderVisual : CompositionDrawListVisual
         }
     }
 
+    public BackgroundSizing BackgroundSizing
+    {
+        get => _backgroundSizing;
+        set
+        {
+            if (_backgroundSizing != value)
+            {
+                _backgroundSizingChanged = true;
+                _backgroundSizing = value;
+                RegisterForSerialization();
+            }
+        }
+    }
+
     private protected override void SerializeChangesCore(BatchStreamWriter writer)
     {
         base.SerializeChangesCore(writer);
@@ -58,12 +74,16 @@ class CompositionBorderVisual : CompositionDrawListVisual
         writer.Write(_borderThicknessChanged);
         if (_borderThicknessChanged)
             writer.Write(_borderThickness);
+        writer.Write(_backgroundSizingChanged);
+        if (_backgroundSizingChanged)
+            writer.Write(_backgroundSizing);
     }
 
     class ServerBorderVisual : ServerCompositionDrawListVisual
     {
         private CornerRadius _cornerRadius;
         private Thickness _borderThickness;
+        private BackgroundSizing _backgroundSizing;
         public ServerBorderVisual(ServerCompositor compositor, Visual v) : base(compositor, v)
         {
         }
@@ -79,6 +99,8 @@ class CompositionBorderVisual : CompositionDrawListVisual
                 _cornerRadius = reader.Read<CornerRadius>();
             if (reader.Read<bool>())
                 _borderThickness = reader.Read<Thickness>();
+            if (reader.Read<bool>())
+                _backgroundSizing = reader.Read<BackgroundSizing>();
         }
 
 

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -447,6 +447,10 @@ namespace Avalonia.Controls.Presenters
                 case nameof(BorderThickness):
                     _layoutThickness = null;
                     break;
+                case nameof(BackgroundSizing):
+                    if (_borderVisual != null)
+                        _borderVisual.BackgroundSizing = BackgroundSizing;
+                    break;
                 case nameof(CornerRadius):
                     if (_borderVisual != null)
                         _borderVisual.CornerRadius = CornerRadius;
@@ -587,6 +591,7 @@ namespace Avalonia.Controls.Presenters
         {
             return _borderVisual = new CompositionBorderVisual(compositor, this)
             {
+                BackgroundSizing = BackgroundSizing,
                 CornerRadius = CornerRadius
             };
         }