Added failing test for #3019.

tests/Avalonia.Visuals.UnitTests/Rendering/DeferredRendererTests.cs

@@ -96,6 +96,55 @@ namespace Avalonia.Visuals.UnitTests.Rendering
             Assert.Equal(new List<IVisual> { root, decorator, border, canvas }, result);
         }
 
+        [Fact]
+        public void Should_Add_Dirty_Rect_On_Child_Remove()
+        {
+            var dispatcher = new ImmediateDispatcher();
+            var loop = new Mock<IRenderLoop>();
+
+            Decorator decorator;
+            Border border;
+            var root = new TestRoot
+            {
+                Width = 100,
+                Height= 100,
+                Child = decorator = new Decorator
+                {
+                    Child = border = new Border
+                    {
+                        Width = 50,
+                        Height = 50,
+                        Background = Brushes.Red,
+                    },
+                }
+            };
+
+            root.Measure(Size.Infinity);
+            root.Arrange(new Rect(root.DesiredSize));
+
+            var sceneBuilder = new SceneBuilder();
+            var target = new DeferredRenderer(
+                root,
+                loop.Object,
+                sceneBuilder: sceneBuilder,
+                dispatcher: dispatcher);
+
+            root.Renderer = target;
+            target.Start();
+            RunFrame(target);
+
+            decorator.Child = null;
+
+            RunFrame(target);
+
+            var scene = target.UnitTestScene();
+            var stackNode = scene.FindNode(decorator);
+            var dirty = scene.Layers[0].Dirty.ToList();
+
+            Assert.Equal(1, dirty.Count);
+            Assert.Equal(new Rect(25, 25, 50, 50), dirty[0]);
+        }
+
         [Fact]
         public void Should_Update_VisualNode_Order_On_Child_Remove_Insert()
         {