Added StyleActivator leak test.

Trying to locate source of StyleActivator leaks - this test doesn't find
it.

src/Perspex.Styling/Styling/Style.cs

@@ -60,13 +60,13 @@ namespace Perspex.Styling
                     var activator = match.ObservableResult ?? 
                         Observable.Never<bool>().StartWith(true);
 
-                    if (visual != null)
-                    {
-                        var detached = Observable.FromEventPattern<VisualTreeAttachmentEventArgs>(
-                            x => visual.DetachedFromVisualTree += x,
-                            x => visual.DetachedFromVisualTree -= x);
-                        activator = activator.TakeUntil(detached);
-                    }
+                    //if (visual != null)
+                    //{
+                    //    var detached = Observable.FromEventPattern<VisualTreeAttachmentEventArgs>(
+                    //        x => visual.DetachedFromVisualTree += x,
+                    //        x => visual.DetachedFromVisualTree -= x);
+                    //    activator = activator.TakeUntil(detached);
+                    //}
 
                     foreach (var setter in Setters)
                     {

tests/Perspex.LeakTests/Perspex.LeakTests.csproj

@@ -88,6 +88,7 @@
     </Reference>
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="StyleTests.cs" />
     <Compile Include="ControlTests.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="TestApp.cs" />

tests/Perspex.LeakTests/StyleTests.cs

@@ -0,0 +1,69 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using JetBrains.dotMemoryUnit;
+using Perspex.Controls;
+using Perspex.Controls.Primitives;
+using Perspex.Controls.Templates;
+using Perspex.Styling;
+using Perspex.VisualTree;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Perspex.LeakTests
+{
+    [DotMemoryUnit(FailIfRunWithoutSupport = false)]
+    public class StyleTests
+    {
+        public StyleTests(ITestOutputHelper atr)
+        {
+            TestApp.Initialize();
+            DotMemoryUnitTestOutput.SetOutputMethod(atr.WriteLine);
+        }
+
+        [Fact]
+        public void StyleActivator_Should_Be_Released()
+        {
+            Func<Window> run = () =>
+            {
+                var window = new Window
+                {
+                    Styles = new Styles
+                    {
+                        new Style(x => x.OfType<Canvas>().Class("foo"))
+                        {
+                            Setters = new[]
+                            {
+                                new Setter(Canvas.WidthProperty, 100),
+                            }
+                        }
+                    },
+                    Content = new Canvas
+                    {
+                        Classes = new Classes("foo"),
+                    }
+                };
+
+                // Do a layout and make sure that styled Canvas gets added to visual tree.
+                window.LayoutManager.ExecuteLayoutPass();
+                Assert.IsType<Canvas>(window.Presenter.Child);
+                Assert.Equal(100, (window.Presenter.Child).Width);
+
+                // Clear the content and ensure the Canvas is removed.
+                window.Content = null;
+                window.LayoutManager.ExecuteLayoutPass();
+                Assert.Null(window.Presenter.Child);
+
+                return window;
+            };
+
+            var result = run();
+
+            dotMemory.Check(memory =>
+                Assert.Equal(0, memory.GetObjects(where => where.Type.Is<StyleActivator>()).ObjectsCount));
+        }
+    }
+}