Don't raise ResourceChanged on reparenting...

...when the new parent isn't attached to the tree. This prevents us sending a load of useless `ResourcesChanged` events.

src/Avalonia.Styling/StyledElement.cs

@@ -418,12 +418,16 @@ namespace Avalonia
                     var e = new LogicalTreeAttachmentEventArgs(newRoot, this, parent);
                     OnAttachedToLogicalTreeCore(e);
                 }
-                else
+                else if (parent is null)
                 {
                     // If we were attached to the logical tree, we piggyback on the tree traversal
                     // there to raise resources changed notifications. If we're being removed from
-                    // the logical tree or attached to a control which is not rooted, then traverse
-                    // the tree raising notifications now.
+                    // the logical tree, then traverse the tree raising notifications now.
+                    //
+                    // We don't raise resources changed notifications if we're being attached to a 
+                    // non-rooted control beacuse it's unlikely that dynamic resources need to be 
+                    // correct until the control is added to the tree, and it causes a *lot* of
+                    // notifications.
                     NotifyResourcesChanged();
                 }
 

tests/Avalonia.Controls.UnitTests/Primitives/TemplatedControlTests.cs

@@ -536,6 +536,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 }),
             };
 
+            var root = new TestRoot(target);
             target.ApplyTemplate();
 
             var contentPresenter = Assert.IsType<ContentPresenter>(target.GetVisualChildren().Single());
@@ -565,6 +566,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 }),
             };
 
+            var root = new TestRoot(target);
             target.ApplyTemplate();
 
             var contentPresenter = Assert.IsType<ContentPresenter>(target.GetVisualChildren().Single());

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/DynamicResourceExtensionTests.cs

@@ -634,33 +634,36 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
         [Fact]
         public void Control_Property_Is_Updated_When_Parent_Is_Changed()
         {
-            var xaml = @"
-<UserControl xmlns='https://github.com/avaloniaui'
+            using (StyledWindow())
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
              xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
-    <UserControl.Resources>
+    <Window.Resources>
         <SolidColorBrush x:Key='brush'>#ff506070</SolidColorBrush>
-    </UserControl.Resources>
+    </Window.Resources>
 
     <Border Name='border' Background='{DynamicResource brush}'/>
-</UserControl>";
+</Window>";
 
-            var loader = new AvaloniaXamlLoader();
-            var userControl = (UserControl)loader.Load(xaml);
-            var border = userControl.FindControl<Border>("border");
+                var loader = new AvaloniaXamlLoader();
+                var window = (Window)loader.Load(xaml);
+                var border = window.FindControl<Border>("border");
 
-            DelayedBinding.ApplyBindings(border);
+                DelayedBinding.ApplyBindings(border);
 
-            var brush = (SolidColorBrush)border.Background;
-            Assert.Equal(0xff506070, brush.Color.ToUint32());
+                var brush = (SolidColorBrush)border.Background;
+                Assert.Equal(0xff506070, brush.Color.ToUint32());
 
-            userControl.Content = null;
+                window.Content = null;
 
-            Assert.Null(border.Background);
+                Assert.Null(border.Background);
 
-            userControl.Content = border;
+                window.Content = border;
 
-            brush = (SolidColorBrush)border.Background;
-            Assert.Equal(0xff506070, brush.Color.ToUint32());
+                brush = (SolidColorBrush)border.Background;
+                Assert.Equal(0xff506070, brush.Color.ToUint32());
+            }
         }
 
         [Fact]

tests/Avalonia.Styling.UnitTests/StyledElementTests.cs

@@ -530,9 +530,9 @@ namespace Avalonia.Styling.UnitTests
         }
 
         [Fact]
-        public void Changing_Parent_Raises_ResourcesChanged()
+        public void Adding_To_Logical_Tree_Raises_ResourcesChanged()
         {
-            var target = new TestControl();
+            var target = new TestRoot();
             var parent = new Decorator { Resources = { { "foo", "bar" } } };
             var raised = 0;