Fix layers with opacity.

We were pushing the opacity twice.

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -28,7 +28,6 @@ namespace Avalonia.Rendering
 
         private bool _running;
         private Scene _scene;
-        private IRenderTarget _renderTarget;
         private DirtyVisuals _dirty;
         private IRenderTargetBitmapImpl _overlay;
         private bool _updateQueued;
@@ -77,7 +76,7 @@ namespace Avalonia.Rendering
             Contract.Requires<ArgumentNullException>(renderTarget != null);
 
             _root = root;
-            _renderTarget = renderTarget;
+            RenderTarget = renderTarget;
             _sceneBuilder = sceneBuilder ?? new SceneBuilder();
             Layers = new RenderLayers();
         }
@@ -98,6 +97,11 @@ namespace Avalonia.Rendering
         /// </summary>
         internal RenderLayers Layers { get; }
 
+        /// <summary>
+        /// Gets the current render target.
+        /// </summary>
+        internal IRenderTarget RenderTarget { get; private set; }
+
         /// <inheritdoc/>
         public void AddDirty(IVisual visual)
         {
@@ -177,9 +181,9 @@ namespace Avalonia.Rendering
             bool renderOverlay = DrawDirtyRects || DrawFps;
             bool composite = false;
 
-            if (_renderTarget == null)
+            if (RenderTarget == null)
             {
-                _renderTarget = ((IRenderRoot)_root).CreateRenderTarget();
+                RenderTarget = ((IRenderRoot)_root).CreateRenderTarget();
             }
 
             if (renderOverlay)
@@ -195,7 +199,7 @@ namespace Avalonia.Rendering
 
                     if (scene.Generation != _lastSceneId)
                     {
-                        context = _renderTarget.CreateDrawingContext(this);
+                        context = RenderTarget.CreateDrawingContext(this);
                         Layers.Update(scene, context);
 
                         RenderToLayers(scene);
@@ -212,13 +216,13 @@ namespace Avalonia.Rendering
 
                     if (renderOverlay)
                     {
-                        context = context ?? _renderTarget.CreateDrawingContext(this);
+                        context = context ?? RenderTarget.CreateDrawingContext(this);
                         RenderOverlay(scene, context);
                         RenderComposite(scene, context);
                     }
                     else if (composite)
                     {
-                        context = context ?? _renderTarget.CreateDrawingContext(this);
+                        context = context ?? RenderTarget.CreateDrawingContext(this);
                         RenderComposite(scene, context);
                     }
 
@@ -228,8 +232,8 @@ namespace Avalonia.Rendering
             catch (RenderTargetCorruptedException ex)
             {
                 Logging.Logger.Information("Renderer", this, "Render target was corrupted. Exception: {0}", ex);
-                _renderTarget?.Dispose();
-                _renderTarget = null;
+                RenderTarget?.Dispose();
+                RenderTarget = null;
             }
         }
 
@@ -241,7 +245,9 @@ namespace Avalonia.Rendering
 
                 if (!clipBounds.IsEmpty && node.Opacity > 0)
                 {
-                    node.BeginRender(context);
+                    var isLayerRoot = node.Visual == layer;
+
+                    node.BeginRender(context, isLayerRoot);
 
                     foreach (var operation in node.DrawOperations)
                     {
@@ -255,7 +261,7 @@ namespace Avalonia.Rendering
                         Render(context, (VisualNode)child, layer, clipBounds);
                     }
 
-                    node.EndRender(context);
+                    node.EndRender(context, isLayerRoot);
                 }
             }
         }

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

@@ -72,13 +72,15 @@ namespace Avalonia.Rendering.SceneGraph
         /// Sets up the drawing context for rendering the node's geometry.
         /// </summary>
         /// <param name="context">The drawing context.</param>
-        void BeginRender(IDrawingContextImpl context);
+        /// <param name="skipOpacity">Whether to skip pushing the control's opacity.</param>
+        void BeginRender(IDrawingContextImpl context, bool skipOpacity);
 
         /// <summary>
         /// Resets the drawing context after rendering the node's geometry.
         /// </summary>
         /// <param name="context">The drawing context.</param>
-        void EndRender(IDrawingContextImpl context);
+        /// <param name="skipOpacity">Whether to skip popping the control's opacity.</param>
+        void EndRender(IDrawingContextImpl context, bool skipOpacity);
 
         /// <summary>
         /// Hit test the geometry in this node.

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

@@ -218,15 +218,15 @@ namespace Avalonia.Rendering.SceneGraph
         }
 
         /// <inheritdoc/>
-        public void BeginRender(IDrawingContextImpl context)
+        public void BeginRender(IDrawingContextImpl context, bool skipOpacity)
         {
             if (ClipToBounds)
             {
                 context.Transform = Matrix.Identity;
                 context.PushClip(ClipBounds);
             }
-
-            if (Opacity != 1)
+            
+            if (Opacity != 1 && !skipOpacity)
             {
                 context.PushOpacity(Opacity);
             }
@@ -240,14 +240,14 @@ namespace Avalonia.Rendering.SceneGraph
         }
 
         /// <inheritdoc/>
-        public void EndRender(IDrawingContextImpl context)
+        public void EndRender(IDrawingContextImpl context, bool skipOpacity)
         {
             if (GeometryClip != null)
             {
                 context.PopGeometryClip();
             }
 
-            if (Opacity != 1)
+            if (Opacity != 1 && !skipOpacity)
             {
                 context.PopOpacity();
             }

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

@@ -201,6 +201,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering
                     {
                         Background = Brushes.Green,
                         Child = new Canvas(),
+                        Opacity = 0.9,
                     }
                 }
             };
@@ -225,6 +226,93 @@ namespace Avalonia.Visuals.UnitTests.Rendering
             Assert.Equal(new[] { root }, target.Layers.Select(x => x.LayerRoot));
         }
 
+        [Fact]
+        public void Should_Not_Create_Layer_For_Childless_Control_With_Animated_Opacity()
+        {
+            Border border;
+            var root = new TestRoot
+            {
+                Width = 100,
+                Height = 100,
+                Child = new Border
+                {
+                    Background = Brushes.Red,
+                    Child = border = new Border
+                    {
+                        Background = Brushes.Green,
+                    }
+                }
+            };
+
+            var animation = new BehaviorSubject<double>(0.5);
+            border.Bind(Border.OpacityProperty, animation, BindingPriority.Animation);
+
+            root.Measure(Size.Infinity);
+            root.Arrange(new Rect(root.DesiredSize));
+
+            var loop = new Mock<IRenderLoop>();
+            var target = CreateTargetAndRunFrame(root, loop: loop);
+
+            Assert.Single(target.Layers);
+        }
+
+        [Fact]
+        public void Should_Not_Push_Opacity_For_Transparent_Layer_Root_Control()
+        {
+            Border border;
+            var root = new TestRoot
+            {
+                Width = 100,
+                Height = 100,
+                Child = border = new Border
+                {
+                    Background = Brushes.Red,
+                    Child = new Canvas(),
+                }
+            };
+
+            var animation = new BehaviorSubject<double>(0.5);
+            border.Bind(Border.OpacityProperty, animation, BindingPriority.Animation);
+
+            root.Measure(Size.Infinity);
+            root.Arrange(new Rect(root.DesiredSize));
+
+            var target = CreateTargetAndRunFrame(root);
+            var context = GetLayerContext(target, border);
+
+            context.Verify(x => x.PushOpacity(0.5), Times.Never);
+            context.Verify(x => x.FillRectangle(Brushes.Red, new Rect(0, 0, 100, 100), 0), Times.Once);
+            context.Verify(x => x.PopOpacity(), Times.Never);
+        }
+
+        [Fact]
+        public void Should_Draw_Transparent_Layer_With_Correct_Opacity()
+        {
+            Border border;
+            var root = new TestRoot
+            {
+                Width = 100,
+                Height = 100,
+                Child = border = new Border
+                {
+                    Background = Brushes.Red,
+                    Child = new Canvas(),
+                }
+            };
+
+            var animation = new BehaviorSubject<double>(0.5);
+            border.Bind(Border.OpacityProperty, animation, BindingPriority.Animation);
+
+            root.Measure(Size.Infinity);
+            root.Arrange(new Rect(root.DesiredSize));
+
+            var target = CreateTargetAndRunFrame(root);
+            var context = Mock.Get(target.RenderTarget.CreateDrawingContext(null));
+            var borderLayer = target.Layers[border].Bitmap;
+
+            context.Verify(x => x.DrawImage(borderLayer, 0.5, It.IsAny<Rect>(), It.IsAny<Rect>()));
+        }
+
         private DeferredRenderer CreateTargetAndRunFrame(
             TestRoot root,
             Mock<IRenderLoop> loop = null,