Add check for correct VisualNode.Parent.

Check that `VisualNode.Parent` is correct when adding a child. Causes a failing test for #3095.

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

@@ -119,6 +119,11 @@ namespace Avalonia.Rendering.SceneGraph
                 throw new ObjectDisposedException("Visual node for {node.Visual}");
             }
 
+            if (child.Parent != this)
+            {
+                throw new AvaloniaInternalException("VisualNode added to wrong parent.");
+            }
+
             EnsureChildrenCreated();
             _children.Add(child);
         }
@@ -155,6 +160,11 @@ namespace Avalonia.Rendering.SceneGraph
                 throw new ObjectDisposedException("Visual node for {node.Visual}");
             }
 
+            if (node.Parent != this)
+            {
+                throw new AvaloniaInternalException("VisualNode added to wrong parent.");
+            }
+
             EnsureChildrenCreated();
             _children[index] = node;
         }

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

@@ -17,7 +17,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
         public void Should_Add_VisualNode()
         {
             var parent = new VisualNode(new TestRoot(), null);
-            var child = new VisualNode(Mock.Of<IVisual>(), null);
+            var child = new VisualNode(Mock.Of<IVisual>(), parent);
             var layers = new SceneLayers(parent.Visual);
             var target = new DeferredDrawingContextImpl(null, layers);
 
@@ -32,7 +32,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
         public void Should_Not_Replace_Identical_VisualNode()
         {
             var parent = new VisualNode(new TestRoot(), null);
-            var child = new VisualNode(Mock.Of<IVisual>(), null);
+            var child = new VisualNode(Mock.Of<IVisual>(), parent);
             var layers = new SceneLayers(parent.Visual);
 
             parent.AddChild(child);
@@ -50,8 +50,8 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
         public void Should_Replace_Different_VisualNode()
         {
             var parent = new VisualNode(new TestRoot(), null);
-            var child1 = new VisualNode(Mock.Of<IVisual>(), null);
+            var child1 = new VisualNode(Mock.Of<IVisual>(), parent);
-            var child2 = new VisualNode(Mock.Of<IVisual>(), null);
+            var child2 = new VisualNode(Mock.Of<IVisual>(), parent);
             var layers = new SceneLayers(parent.Visual);
 
             parent.AddChild(child1);
@@ -78,8 +78,8 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
 
             var layers = new SceneLayers(root);
             var target = new DeferredDrawingContextImpl(null, layers);
-            var child1 = new VisualNode(Mock.Of<IVisual>(), null) { LayerRoot = root };
+            var child1 = new VisualNode(Mock.Of<IVisual>(), node) { LayerRoot = root };
-            var child2 = new VisualNode(Mock.Of<IVisual>(), null) { LayerRoot = root };
+            var child2 = new VisualNode(Mock.Of<IVisual>(), node) { LayerRoot = root };
 
             target.BeginUpdate(node);
             using (target.BeginUpdate(child1)) { }

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

@@ -24,7 +24,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
             var node = new VisualNode(Mock.Of<IVisual>(), null);
             var collection = node.Children;
 
-            node.AddChild(Mock.Of<IVisualNode>());
+            node.AddChild(Mock.Of<IVisualNode>(x => x.Parent == node));
 
             Assert.NotSame(collection, node.Children);
         }
@@ -106,9 +106,9 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
         public void TrimChildren_Should_Work_Correctly()
         {
             var parent = new VisualNode(Mock.Of<IVisual>(), null);
-            var child1 = new VisualNode(Mock.Of<IVisual>(), null);
+            var child1 = new VisualNode(Mock.Of<IVisual>(), parent);
-            var child2 = new VisualNode(Mock.Of<IVisual>(), null);
+            var child2 = new VisualNode(Mock.Of<IVisual>(), parent);
-            var child3 = new VisualNode(Mock.Of<IVisual>(), null);
+            var child3 = new VisualNode(Mock.Of<IVisual>(), parent);
 
             parent.AddChild(child1);
             parent.AddChild(child2);