Fix headless FontManager leak (#19599)

* Dispose font manager after headless session

* Add xunit/nunit LeakTests

src/Headless/Avalonia.Headless/HeadlessUnitTestSession.cs

@@ -5,6 +5,8 @@ using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 using System.Threading;
 using System.Threading.Tasks;
+using Avalonia.Controls;
+using Avalonia.Media;
 using Avalonia.Metadata;
 using Avalonia.Reactive;
 using Avalonia.Threading;
@@ -124,9 +126,10 @@ public sealed class HeadlessUnitTestSession : IDisposable
     private IDisposable EnsureApplication()
     {
         var scope = AvaloniaLocator.EnterScope();
+        var oldContext = SynchronizationContext.Current;
         try
         {
-            Dispatcher.ResetForUnitTests();
+            Dispatcher.ResetBeforeUnitTests();
             _appBuilder.SetupUnsafe();
         }
         catch
@@ -137,9 +140,12 @@ public sealed class HeadlessUnitTestSession : IDisposable
 
         return Disposable.Create(() =>
         {
-            scope.Dispose();
+            ((ToolTipService?)AvaloniaLocator.Current.GetService<IToolTipService>())?.Dispose();
+            (AvaloniaLocator.Current.GetService<FontManager>() as IDisposable)?.Dispose();
             Dispatcher.ResetForUnitTests();
-            SynchronizationContext.SetSynchronizationContext(null);
+            scope.Dispose();
+            Dispatcher.ResetBeforeUnitTests();
+            SynchronizationContext.SetSynchronizationContext(oldContext);
         });
     }
 

tests/Avalonia.Headless.UnitTests/LeakTests.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Media;
+
+namespace Avalonia.Headless.UnitTests;
+
+public class LeakTests
+{
+    public static IEnumerable<object[]> TestData { get; } =
+        Enumerable.Range(0, 50).Select(i => new object[] { i.ToString() });
+
+    private static WeakReference s_previousFontManager;
+
+#if NUNIT
+    [TestCaseSource(nameof(TestData))]
+    [AvaloniaTest, Timeout(10000)]
+#elif XUNIT
+    [MemberData(nameof(TestData))]
+    [AvaloniaTheory]
+#endif
+    public void Previous_FontManager_Should_Be_Collected(string data)
+    {
+        // Arrange
+        var fontManager = new WeakReference(FontManager.Current);
+        var button = new Button { Content = data };
+        var window = new Window
+        {
+            Content = button
+        };
+
+        // Act, just some interaction, to make sure the FontManager is actually used
+        window.Show();
+        button.Focus();
+        window.MouseDown(new Point(1,1), MouseButton.Left);
+        window.Close();
+
+        // Assert
+        GC.Collect();
+        GC.WaitForPendingFinalizers();
+        GC.Collect();
+
+        if (s_previousFontManager is not null)
+        {
+            Assert.False(s_previousFontManager.IsAlive);
+        }
+
+        s_previousFontManager = fontManager;
+    }
+}