Implemented ElementComposition.SetElementChildVisual

src/Avalonia.Base/Rendering/Composition/CompositingRenderer.cs

@@ -146,8 +146,18 @@ public class CompositingRenderer : IRendererWithCompositor
                 return result == 0 ? lhs.index.CompareTo(rhs.index) : result;
             });
         }
-
-        if (compositionChildren.Count == visualChildren.Count)
+        
+        var childVisual = v.ChildCompositionVisual;
+        
+        // Check if the current visual somehow got migrated to another compositor
+        if (childVisual != null && childVisual.Compositor != v.CompositionVisual.Compositor)
+            childVisual = null;
+        
+        var expectedCount = visualChildren.Count;
+        if (childVisual != null)
+            expectedCount++;
+        
+        if (compositionChildren.Count == expectedCount)
         {
             bool mismatch = false;
             if (sortedChildren != null)
@@ -167,6 +177,9 @@ public class CompositingRenderer : IRendererWithCompositor
                         break;
                     }
 
+            if (childVisual != null &&
+                !ReferenceEquals(compositionChildren[compositionChildren.Count - 1], childVisual))
+                mismatch = true;
 
             if (!mismatch)
             {
@@ -193,6 +206,9 @@ public class CompositingRenderer : IRendererWithCompositor
                 if (compositionChild != null)
                     compositionChildren.Add(compositionChild);
             }
+
+        if (childVisual != null)
+            compositionChildren.Add(childVisual);
     }
 
     private void UpdateCore()

src/Avalonia.Base/Rendering/Composition/Compositor.Factories.cs

@@ -29,4 +29,7 @@ public partial class Compositor
     public ImplicitAnimationCollection CreateImplicitAnimationCollection() => new ImplicitAnimationCollection(this);
 
     public CompositionAnimationGroup CreateAnimationGroup() => new CompositionAnimationGroup(this);
+
+    public CompositionSolidColorVisual CreateSolidColorVisual() =>
+        new(this, new ServerCompositionSolidColorVisual(Server));
 }

src/Avalonia.Base/Rendering/Composition/ElementCompositionPreview.cs

@@ -1,5 +1,8 @@
 // Special license applies <see href="https://raw.githubusercontent.com/AvaloniaUI/Avalonia/master/src/Avalonia.Base/Rendering/Composition/License.md">License.md</see>
 
+using System;
+using Avalonia.VisualTree;
+
 namespace Avalonia.Rendering.Composition;
 
 /// <summary>
@@ -13,4 +16,22 @@ public static class ElementComposition
     /// <param name="visual"></param>
     /// <returns></returns>
     public static CompositionVisual? GetElementVisual(Visual visual) => visual.CompositionVisual;
+
+    /// <summary>
+    /// Sets a custom <see cref="CompositionVisual"/> as the last child of the element’s visual tree.
+    /// </summary>
+    public static void SetElementChildVisual(Visual visual, CompositionVisual? compositionVisual)
+    {
+        if (compositionVisual != null && visual.CompositionVisual != null &&
+            compositionVisual.Compositor != visual.CompositionVisual.Compositor)
+            throw new InvalidOperationException("Composition visuals belong to different compositor instances");
+        
+        visual.ChildCompositionVisual = compositionVisual;
+        visual.GetVisualRoot()?.Renderer.RecalculateChildren(visual);
+    }
+
+    /// <summary>
+    /// Retrieves a <see cref="CompositionVisual"/> object previously set by a call to <see cref="SetElementChildVisual" />.
+    /// </summary>
+    public static CompositionVisual? GetElementChildVisual(Visual visual) => visual.ChildCompositionVisual;
 }

src/Avalonia.Base/Rendering/Composition/Server/ServerCompositionSolidColorVisual.cs

@@ -0,0 +1,11 @@
+using Avalonia.Media.Immutable;
+
+namespace Avalonia.Rendering.Composition.Server;
+
+internal partial class ServerCompositionSolidColorVisual
+{
+    protected override void RenderCore(CompositorDrawingContextProxy canvas, Rect currentTransformedClip)
+    {
+        canvas.DrawRectangle(new ImmutableSolidColorBrush(Color), null, new Rect(0, 0, Size.X, Size.Y));
+    }
+}

src/Avalonia.Base/Visual.cs

@@ -292,6 +292,7 @@ namespace Avalonia
         protected IRenderRoot? VisualRoot => _visualRoot ?? (this as IRenderRoot);
 
         internal CompositionDrawListVisual? CompositionVisual { get; private set; }
+        internal CompositionVisual? ChildCompositionVisual { get; set; }
         
         public bool HasNonUniformZIndexChildren { get; private set; }
 

src/Avalonia.Base/composition-schema.xml

@@ -27,6 +27,9 @@
         <Property Name="OpacityMaskBrush" Type="Avalonia.Media.IBrush?" Internal="true" />
     </Object>
     <Object Name="CompositionContainerVisual" Inherits="CompositionVisual"/>
+    <Object Name="CompositionSolidColorVisual" Inherits="CompositionContainerVisual">
+        <Property Name="Color" Type="Avalonia.Media.Color" Animated="true" />
+    </Object>
     <List Name="CompositionVisualCollection" ItemType="CompositionVisual" CustomCtor="true"/>
     <Object Name="CompositionTarget" CustomServerCtor="true">
         <Property Name="Root" Type="CompositionVisual?"/>