Ensure control is invalidated on ZIndex change.

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -561,7 +561,6 @@ namespace Avalonia.Rendering
                         }
                     }
 
-                    System.Diagnostics.Debug.WriteLine("Invalidated " + rect);
                     SceneInvalidated(this, new SceneInvalidatedEventArgs((IRenderRoot)_root, rect));
                 }
             }

src/Avalonia.Visuals/Visual.cs

@@ -514,7 +514,9 @@ namespace Avalonia
         /// <param name="e">The event args.</param>
         private static void ZIndexChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var parent = (e.Sender as Visual)?._visualParent;
+            var sender = e.Sender as IVisual;
+            var parent = sender?.VisualParent;
+            sender?.InvalidateVisual();
             parent?.VisualRoot?.Renderer?.RecalculateChildren(parent);
         }
 

tests/Avalonia.Visuals.UnitTests/VisualTests.cs

@@ -282,5 +282,52 @@ namespace Avalonia.Visuals.UnitTests
 
             Assert.True(called);
         }
+
+        [Fact]
+        public void Changing_ZIndex_Should_InvalidateVisual()
+        {
+            Canvas canvas1;
+            var renderer = new Mock<IRenderer>();
+            var root = new TestRoot
+            {
+                Child = new StackPanel
+                {
+                    Children =
+                    {
+                        (canvas1 = new Canvas()),
+                        new Canvas(),
+                    },
+                },
+            };
+
+            root.Renderer = renderer.Object;
+            canvas1.ZIndex = 10;
+
+            renderer.Verify(x => x.AddDirty(canvas1));
+        }
+
+        [Fact]
+        public void Changing_ZIndex_Should_Recalculate_Parent_Children()
+        {
+            Canvas canvas1;
+            StackPanel stackPanel;
+            var renderer = new Mock<IRenderer>();
+            var root = new TestRoot
+            {
+                Child = stackPanel = new StackPanel
+                {
+                    Children =
+                    {
+                        (canvas1 = new Canvas()),
+                        new Canvas(),
+                    },
+                },
+            };
+
+            root.Renderer = renderer.Object;
+            canvas1.ZIndex = 10;
+
+            renderer.Verify(x => x.RecalculateChildren(stackPanel));
+        }
     }
 }