Revert CancellationToken changes

This reverts commit 6b51f00a4a.

src/Headless/Avalonia.Headless/HeadlessUnitTestSession.cs

@@ -83,13 +83,14 @@ public sealed class HeadlessUnitTestSession : IDisposable
             using var application = EnsureApplication();
 
             var cts = new CancellationTokenSource();
-            using var globalCts = token.Register(s => ((CancellationTokenSource)s!).Cancel(), cts);
-            using var localCts = cancellationToken.Register(s => ((CancellationTokenSource)s!).Cancel(), cts);
+            using var globalCts = token.Register(s => ((CancellationTokenSource)s!).Cancel(), cts, true);
+            using var localCts = cancellationToken.Register(s => ((CancellationTokenSource)s!).Cancel(), cts, true);
 
             try
             {
                 var task = action();
-                task.ContinueWith((_, s) => ((CancellationTokenSource)s!).Cancel(), cts);
+                task.ContinueWith((_, s) => ((CancellationTokenSource)s!).Cancel(), cts,
+                    TaskScheduler.FromCurrentSynchronizationContext());
 
                 if (cts.IsCancellationRequested)
                 {
@@ -97,7 +98,7 @@ public sealed class HeadlessUnitTestSession : IDisposable
                 }
 
                 var frame = new DispatcherFrame();
-                using var innerCts = cts.Token.Register(() => frame.Continue = false);
+                using var innerCts = cts.Token.Register(() => frame.Continue = false, true);
                 Dispatcher.UIThread.PushFrame(frame);
 
                 var result = task.GetAwaiter().GetResult();