Break layout flip flop in headless

src/Headless/Avalonia.Headless/HeadlessWindowImpl.cs

@@ -114,23 +114,26 @@ namespace Avalonia.Headless
         public Size MaxClientSize { get; } = new Size(1920, 1280);
         public void Resize(Size clientSize, WindowResizeReason reason)
         {
+            if (ClientSize == clientSize)
+                return;
+
             // Emulate X11 behavior here
             if (IsPopup)
-                DoResize(clientSize);
+                DoResize(clientSize, reason);
             else
                 Dispatcher.UIThread.Post(() =>
                 {
-                    DoResize(clientSize);
+                    DoResize(clientSize, reason);
-                });
+                }, DispatcherPriority.Send);
         }
 
-        private void DoResize(Size clientSize)
+        private void DoResize(Size clientSize, WindowResizeReason reason)
         {
             // Uncomment this check and experience a weird bug in layout engine
             if (ClientSize != clientSize)
             {
                 ClientSize = clientSize;
-                Resized?.Invoke(clientSize, WindowResizeReason.Unspecified);
+                Resized?.Invoke(clientSize, reason);
             }
         }
 

tests/Avalonia.Headless.UnitTests/RenderingTests.cs

@@ -1,4 +1,5 @@
-using Avalonia.Controls;
+using System.Collections.ObjectModel;
+using Avalonia.Controls;
 using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.Threading;
@@ -24,6 +25,27 @@ public class RenderingTests
                 Content = new PathIcon
                 {
                     Data = StreamGeometry.Parse("M0,9 L10,0 20,9 19,10 10,2 1,10 z")
+#if NUNIT
+    [AvaloniaTest, Timeout(10000)]
+#elif XUNIT
+    [AvaloniaFact(Timeout = 10000)]
+#endif
+    public void Should_Not_Hang_With_Non_Trivial_Layout()
+    {
+        var window = new Window
+        {
+            Content = new ContentControl
+            {
+                HorizontalAlignment = HorizontalAlignment.Stretch,
+                VerticalAlignment = VerticalAlignment.Stretch,
+                Padding = new Thickness(1),
+                Content = new ListBox
+                {
+                    ItemsSource = new ObservableCollection<string>()
+                    {
+                        "Test 1",
+                        "Test 2"
+                    }
                 }
             },
             SizeToContent = SizeToContent.WidthAndHeight