Introduced counting of rendered visuals in ServerCompositionVisual and added relevant unit tests in CompositorInvalidationClippingTests. The new tests ensure that visuals that are not in dirty rect are rendered correctly with different ClipToBounds and Clip geometry parameters.

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

@@ -2,5 +2,7 @@ namespace Avalonia.Rendering.Composition;
 
 internal interface ICompositionTargetDebugEvents
 {
+    public int RenderedVisuals { get; }
+    void IncrementRenderedVisuals();
     void RectInvalidated(Rect rc);
 }

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

@@ -38,6 +38,7 @@ namespace Avalonia.Rendering.Composition.Server
                 return;
 
             Root!.RenderedVisuals++;
+            Root!.DebugEvents?.IncrementRenderedVisuals();
 
             var boundsRect = new Rect(new Size(Size.X, Size.Y));
 

tests/Avalonia.Base.UnitTests/Rendering/CompositorInvalidationClippingTests.cs

@@ -0,0 +1,60 @@
+using Avalonia.Controls;
+using Avalonia.Media;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Rendering;
+
+public class CompositorInvalidationClippingTests : CompositorTestsBase
+{
+    [Fact]
+    public void Siblings_Should_Be_Rendered_On_Invalidate_Without_ClipToBounds()
+    {
+        AssertRenderedVisuals(clipToBounds: false, clipGeometry: false, expectedRenderedVisualsCount: 4);
+    }
+
+    [Fact]
+    public void Siblings_Should_Not_Be_Rendered_On_Invalidate_With_ClipToBounds()
+    {
+        AssertRenderedVisuals(clipToBounds: true, clipGeometry: false, expectedRenderedVisualsCount: 3);
+    }
+
+    [Fact]
+    public void Siblings_Should_Not_Be_Rendered_On_Invalidate_With_Clip()
+    {
+        AssertRenderedVisuals(clipToBounds: false, clipGeometry: true, expectedRenderedVisualsCount: 3);
+    }
+
+    private void AssertRenderedVisuals(bool clipToBounds, bool clipGeometry, int expectedRenderedVisualsCount)
+    {
+        using (var s = new CompositorCanvas())
+        {
+            //#1 visual to render is root
+            //#2 visual to render is s.Canvas
+            
+            //#3 visual to render
+            s.Canvas.Children.Add(new Border()
+            {
+                [Canvas.LeftProperty] = 0, [Canvas.TopProperty] = 0,
+                Width = 20, Height = 10,
+                Background = Brushes.Red,
+                ClipToBounds = clipToBounds,
+                Clip = clipGeometry ? new RectangleGeometry(new Rect(new Size(20, 10))) : null
+            });
+            
+            //#4 visual to render
+            s.Canvas.Children.Add(new Border()
+            {
+                [Canvas.LeftProperty] = 30, [Canvas.TopProperty] = 50,
+                Width = 20, Height = 10,
+                Background = Brushes.Red,
+                ClipToBounds = clipToBounds,
+                Clip = clipGeometry ? new RectangleGeometry(new Rect(new Size(20, 10))) : null
+            });
+            s.RunJobs();
+            s.Events.Reset();
+            s.Canvas.Children[0].IsVisible = false;
+            s.RunJobs();
+            s.AssertRenderedVisuals(expectedRenderedVisualsCount);
+        }
+    }
+}

tests/Avalonia.UnitTests/CompositorTestServices.cs

@@ -89,6 +89,13 @@ public class CompositorTestServices : IDisposable
         Events.Rects.Clear();
     }
 
+    public void AssertRenderedVisuals(int renderVisuals)
+    {
+        RunJobs();
+        Assert.Equal(Events.RenderedVisuals, renderVisuals);
+        Events.Rects.Clear();
+    }
+
     public void AssertHitTest(double x, double y, Func<Visual, bool> filter, params object[] expected)
         => AssertHitTest(new Point(x, y), filter, expected);
 
@@ -110,6 +117,13 @@ public class CompositorTestServices : IDisposable
     {
         public List<Rect> Rects = new();
 
+        public int RenderedVisuals { get; private set; }
+
+        public void IncrementRenderedVisuals()
+        {
+            RenderedVisuals++;
+        }
+
         public void RectInvalidated(Rect rc)
         {
             Rects.Add(rc);
@@ -118,6 +132,7 @@ public class CompositorTestServices : IDisposable
         public void Reset()
         {
             Rects.Clear();
+            RenderedVisuals = 0;
         }
     }
 
@@ -218,4 +233,4 @@ public class DispatcherCompositorScheduler : ICompositorScheduler
     {
         Dispatcher.UIThread.Post(() => compositor.Commit(), DispatcherPriority.UiThreadRender);
     }
-}
+}