Merge pull request #3118 from AvaloniaUI/fixes/3115-visualnode-disposed

Add VisualNode to scene even for invisible controls

src/Avalonia.Visuals/Rendering/SceneGraph/SceneBuilder.cs

@@ -240,6 +240,10 @@ namespace Avalonia.Rendering.SceneGraph
                     }
                 }
             }
+            else
+            {
+                contextImpl.BeginUpdate(node).Dispose();
+            }
         }
 
         private void UpdateSize(Scene scene)

src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs

@@ -218,7 +218,7 @@ namespace Avalonia.Rendering.SceneGraph
             if (first < _children?.Count)
             {
                 EnsureChildrenCreated();
-                for (int i = first; i < _children.Count - first; i++)
+                for (int i = first; i < _children.Count; i++)
                 {
                     _children[i].Dispose();
                 }

tests/Avalonia.Visuals.UnitTests/Rendering/SceneGraph/SceneBuilderTests.cs

@@ -577,6 +577,59 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
             }
         }
 
+        [Fact]
+        public void Should_Not_Dispose_Active_VisualNode_When_Control_Reparented_And_Child_Made_Invisible()
+        {
+            // Issue #3115
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                StackPanel panel;
+                Border border1;
+                Border border2;
+                var tree = new TestRoot
+                {
+                    Width = 100,
+                    Height = 100,
+                    Child = panel = new StackPanel
+                    {
+                        Children =
+                        {
+                            (border1 = new Border
+                            {
+                                Background = Brushes.Red,
+                            }),
+                            (border2 = new Border
+                            {
+                                Background = Brushes.Green,
+                            }),
+                        }
+                    }
+                };
+
+                tree.Measure(Size.Infinity);
+                tree.Arrange(new Rect(tree.DesiredSize));
+
+                var scene = new Scene(tree);
+                var sceneBuilder = new SceneBuilder();
+                sceneBuilder.UpdateAll(scene);
+
+                var decorator = new Decorator();
+                tree.Child = null;
+                decorator.Child = panel;
+                tree.Child = decorator;
+                border1.IsVisible = false;
+
+                scene = scene.CloneScene();
+
+                var panelNode = (VisualNode)scene.FindNode(panel);
+                sceneBuilder.Update(scene, decorator);
+
+                Assert.Equal(2, panelNode.Children.Count);
+                Assert.False(panelNode.Children[0].Disposed);
+                Assert.False(panelNode.Children[1].Disposed);
+            }
+        }
+
         [Fact]
         public void Should_Update_ClipBounds_For_Negative_Margin()
         {

tests/Avalonia.Visuals.UnitTests/Rendering/SceneGraph/VisualNodeTests.cs

@@ -101,5 +101,24 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
 
             node.SortChildren(scene);
         }
+
+        [Fact]
+        public void TrimChildren_Should_Work_Correctly()
+        {
+            var parent = new VisualNode(Mock.Of<IVisual>(), null);
+            var child1 = new VisualNode(Mock.Of<IVisual>(), null);
+            var child2 = new VisualNode(Mock.Of<IVisual>(), null);
+            var child3 = new VisualNode(Mock.Of<IVisual>(), null);
+
+            parent.AddChild(child1);
+            parent.AddChild(child2);
+            parent.AddChild(child3);
+            parent.TrimChildren(2);
+
+            Assert.Equal(2, parent.Children.Count);
+            Assert.False(child1.Disposed);
+            Assert.False(child2.Disposed);
+            Assert.True(child3.Disposed);
+        }
     }
 }