Add ref-counting for the scene instance in DeferredRenderer to ensure that we don't dispose a scene while it is being rendered.

Also rename Scene.Clone to Scene.CloneScene to prevent confusion between IRef<Scene>.Clone and Scene.Clone.

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Rendering
         private readonly ISceneBuilder _sceneBuilder;
 
         private bool _running;
-        private Scene _scene;
+        private volatile IRef<Scene> _scene;
         private DirtyVisuals _dirty;
         private IRef<IRenderTargetBitmapImpl> _overlay;
         private bool _updateQueued;
@@ -128,7 +128,7 @@ namespace Avalonia.Rendering
                 UpdateScene();
             }
 
-            return _scene?.HitTest(p, root, filter) ?? Enumerable.Empty<IVisual>();
+            return _scene?.Item.HitTest(p, root, filter) ?? Enumerable.Empty<IVisual>();
         }
 
         /// <inheritdoc/>
@@ -180,7 +180,7 @@ namespace Avalonia.Rendering
 
         internal void UnitTestUpdateScene() => UpdateScene();
 
-        internal void UnitTestRender() => Render(_scene);
+        internal void UnitTestRender() => Render(_scene.Item);
 
         private void Render(Scene scene)
         {
@@ -381,7 +381,8 @@ namespace Avalonia.Rendering
             {
                 if (_root.IsVisible)
                 {
-                    var scene = _scene?.Clone() ?? new Scene(_root);
+                    var sceneRef = RefCountable.Create(_scene?.Item.CloneScene() ?? new Scene(_root));
+                    var scene = sceneRef.Item;
 
                     if (_dirty == null)
                     {
@@ -396,7 +397,7 @@ namespace Avalonia.Rendering
                         }
                     }
 
-                    var oldScene = Interlocked.Exchange(ref _scene, scene);
+                    var oldScene = Interlocked.Exchange(ref _scene, sceneRef);
                     oldScene?.Dispose();
 
                     _dirty.Clear();
@@ -426,9 +427,18 @@ namespace Avalonia.Rendering
                         _dispatcher.Post(UpdateScene, DispatcherPriority.Render);
                     }
 
-                    Scene scene = null;
+                    var scene = _scene?.Clone();
-                    Interlocked.Exchange(ref scene, _scene);
+                    if (scene == null)
-                    Render(scene);
+                    {
+                        Render(null);
+                    }
+                    else
+                    {
+                        using (scene)
+                        {
+                            Render(scene.Item);
+                        } 
+                    }
                 }
                 catch { }
                 finally

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

@@ -82,7 +82,7 @@ namespace Avalonia.Rendering.SceneGraph
         /// Clones the scene.
         /// </summary>
         /// <returns>The cloned scene.</returns>
-        public Scene Clone()
+        public Scene CloneScene()
         {
             var index = new Dictionary<IVisual, IVisualNode>();
             var root = Clone((VisualNode)Root, null, index);

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

@@ -99,7 +99,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 Assert.Equal(new Rect(10, 20, 160, 240), canvasNode.ClipBounds);
 
                 // Initial ClipBounds are correct, make sure they're still correct after updating canvas.
-                result = result.Clone();
+                result = result.CloneScene();
                 Assert.True(sceneBuilder.Update(result, canvas));
 
                 canvasNode = result.FindNode(canvas);
@@ -197,7 +197,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 canvas.Arrange(new Rect(tree.DesiredSize));
 
                 // Initial ClipBounds are correct, make sure they're still correct after updating canvas.
-                scene = scene.Clone();
+                scene = scene.CloneScene();
                 Assert.True(sceneBuilder.Update(scene, canvas));
 
                 borderNode = scene.FindNode(border);
@@ -309,7 +309,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 var borderNode = scene.FindNode(border);
                 Assert.Equal(expectedTransform, borderNode.Transform);
 
-                scene = scene.Clone();
+                scene = scene.CloneScene();
                 Assert.True(sceneBuilder.Update(scene, border));
 
                 borderNode = scene.FindNode(border);
@@ -354,7 +354,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
 
                 border.Background = Brushes.Green;
 
-                var result = initial.Clone();
+                var result = initial.CloneScene();
                 sceneBuilder.Update(result, border);
                 
                 var borderNode = (VisualNode)result.Root.Children[0];
@@ -402,7 +402,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 sceneBuilder.UpdateAll(initial);
 
                 border.Child = decorator;
-                var result = initial.Clone();
+                var result = initial.CloneScene();
 
                 Assert.True(sceneBuilder.Update(result, decorator));
 
@@ -457,7 +457,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 sceneBuilder.UpdateAll(initial);
 
                 border.Child = null;
-                var result = initial.Clone();
+                var result = initial.CloneScene();
 
                 Assert.True(sceneBuilder.Update(result, decorator));
                 Assert.False(sceneBuilder.Update(result, canvas));
@@ -501,7 +501,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 sceneBuilder.UpdateAll(initial);
 
                 border.IsVisible = false;
-                var result = initial.Clone();
+                var result = initial.CloneScene();
 
                 Assert.True(sceneBuilder.Update(result, border));
                 Assert.False(sceneBuilder.Update(result, canvas));
@@ -552,7 +552,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 decorator.Margin = new Thickness(0, 20, 0, 0);
                 layout.ExecuteLayoutPass();
 
-                scene = scene.Clone();
+                scene = scene.CloneScene();
                 sceneBuilder.Update(scene, decorator);
 
                 borderNode = scene.FindNode(border);
@@ -600,7 +600,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 decorator.Margin = new Thickness(0, 20, 0, 0);
                 layout.ExecuteLayoutPass();
 
-                scene = scene.Clone();
+                scene = scene.CloneScene();
 
                 sceneBuilder.Update(scene, decorator);
 
@@ -640,7 +640,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 Assert.Equal(new Size(100, 100), scene.Size);
 
                 tree.ClientSize = new Size(110, 120);
-                scene = scene.Clone();
+                scene = scene.CloneScene();
                 sceneBuilder.Update(scene, tree);
 
                 Assert.Equal(new Size(110, 120), scene.Size);
@@ -735,7 +735,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
 
                 tree.Child = new Decorator();
 
-                using (var result = scene.Clone())
+                using (var result = scene.CloneScene())
                 {
                     sceneBuilder.Update(result, img);
                     scene.Dispose();

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

@@ -62,7 +62,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 Assert.Empty(scene.Layers.Select(x => x.LayerRoot).Except(new IVisual[] { tree, border }));
 
                 animation.OnCompleted();
-                scene = scene.Clone();
+                scene = scene.CloneScene();
 
                 sceneBuilder.Update(scene, border);
 
@@ -160,7 +160,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 Assert.Equal(3, scene.Layers.Count);
 
                 decorator.Child = null;
-                scene = scene.Clone();
+                scene = scene.CloneScene();
 
                 sceneBuilder.Update(scene, border);
 
@@ -210,7 +210,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 Assert.Equal(3, scene.Layers.Count);
 
                 border.IsVisible = false;
-                scene = scene.Clone();
+                scene = scene.CloneScene();
 
                 sceneBuilder.Update(scene, border);
 

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

@@ -25,7 +25,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
             scene.Layers[tree].Dirty.Add(new Rect(0, 0, 100, 100));
             scene.Layers[decorator].Dirty.Add(new Rect(0, 0, 50, 100));
 
-            scene = scene.Clone();
+            scene = scene.CloneScene();
             Assert.Equal(2, scene.Layers.Count());
             Assert.Empty(scene.Layers[0].Dirty);
             Assert.Empty(scene.Layers[1].Dirty);