fix(macos): prevent title bar flash on fullscreen exit

- Restore title bar transparency before AppKit begins the exit animation for windows with an extended client area.

Author: Illustar0

native/Avalonia.Native/src/OSX/AvnWindow.mm

@@ -348,6 +348,10 @@ - (void)windowDidResize:(NSNotification *_Nonnull)notification
 
 - (void)windowWillExitFullScreen:(NSNotification *_Nonnull)notification
 {
+    // Prepare the destination appearance before AppKit starts the exit animation.
+    if (_isExtended)
+        [self setTitlebarAppearsTransparent:true];
+
     auto parent = _parent.tryGetWithCast<IWindowStateChanged>();
 
     if(parent != nullptr)