Fixed dispatcher frame capturing wrong dispatcher (#22086)

src/Avalonia.Base/Threading/DispatcherFrame.cs

@@ -38,7 +38,7 @@ public class DispatcherFrame
     ///        for their important criteria to be met.  These frames
     ///        should have a timeout associated with them.
     /// </param>
-    public DispatcherFrame(bool exitWhenRequested) : this(Dispatcher.UIThread, exitWhenRequested)
+    public DispatcherFrame(bool exitWhenRequested) : this(Dispatcher.CurrentDispatcher, exitWhenRequested)
     {
         Dispatcher.VerifyAccess();
     }
@@ -139,4 +139,4 @@ public class DispatcherFrame
         if (_exitWhenRequested)
             _cancellationTokenSource?.Cancel();
     }
-}
+}

tests/Avalonia.Base.UnitTests/DispatcherTests.cs

@@ -317,6 +317,21 @@ public partial class DispatcherTests
         }
     }
 
+    [Fact]
+    public async Task DispatcherFrame_Uses_Current_Dispatcher()
+    {
+        var uiThreadDispatcher = Dispatcher.UIThread;
+
+        await ThreadRunHelper.RunOnDedicatedThread(() =>
+        {
+            var currentDispatcher = Dispatcher.CurrentDispatcher;
+            var frame = new DispatcherFrame();
+
+            Assert.NotSame(uiThreadDispatcher, currentDispatcher);
+            Assert.Same(currentDispatcher, frame.Dispatcher);
+        });
+    }
+
     [Fact]
     public void ExitAllFramesShouldExitAllFramesAndBeAbleToContinue()
     {