Do not use visual children enumerator in TopLevelHost (#21048)

* Add failing test for ExtendClientAreaToDecorationsHint in AttachedToVisualTree

* Do not use visual children enumerator in TopLevelHost

src/Avalonia.Controls/TopLevelHost.cs

@@ -51,11 +51,14 @@ internal partial class TopLevelHost : Control
         var hasInset = inset != default;
         var desiredSize = default(Size);
 
-        foreach (var child in VisualChildren)
+        var children = VisualChildren;
+        var childrenCount = children.Count;
+
+        for (var i = 0; i < childrenCount; i++)
         {
-            if (child is Layoutable l)
+            if (children[i] is Layoutable l)
             {
-                if (hasInset && ReferenceEquals(child, _topLevel))
+                if (hasInset && ReferenceEquals(l, _topLevel))
                 {
                     // In forced mode, measure the TopLevel with reduced size
                     var contentSize = new Size(
@@ -89,11 +92,14 @@ internal partial class TopLevelHost : Control
         var inset = _decorationInset;
         var hasInset = inset != default;
 
-        foreach (var child in VisualChildren)
+        var children = VisualChildren;
+        var childrenCount = children.Count;
+
+        for (var i = 0; i < childrenCount; i++)
         {
-            if (child is Layoutable l)
+            if (children[i] is Layoutable l)
             {
-                if (hasInset && ReferenceEquals(child, _topLevel))
+                if (hasInset && ReferenceEquals(l, _topLevel))
                 {
                     // In forced mode, arrange the TopLevel within the inset area
                     var contentSize = new Size(

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -710,6 +710,35 @@ namespace Avalonia.Controls.UnitTests
             Assert.False(visualRoot.HasMirrorTransform);
         }
 
+        [Fact]
+        public void Extending_Client_Area_To_Decorations_When_Attached_To_Visual_Tree_Works()
+        {
+            var extended = false;
+
+            var windowImpl = MockWindowingPlatform.CreateWindowMock();
+            windowImpl.Setup(w => w.NeedsManagedDecorations).Returns(() => extended);
+            windowImpl.Setup(w => w.RequestedDrawnDecorations).Returns(PlatformRequestedDrawnDecoration.TitleBar);
+
+            using var app = UnitTestApplication.Start(TestServices.StyledWindow.With(
+                windowingPlatform: new MockWindowingPlatform(() => windowImpl.Object)));
+
+            var border = new Border();
+
+            var window = new Window
+            {
+                Content = border
+            };
+
+            border.AttachedToVisualTree +=
+                (_, _) =>
+                {
+                    extended = true;
+                    windowImpl.Object.ExtendClientAreaToDecorationsChanged?.Invoke(true);
+                };
+
+            window.Show();
+        }
+
         public class SizingTests : ScopedTestBase
         {
             [Fact]