https://bugs.winehq.org/show_bug.cgi?id=50717
--- Comment #6 from tempel.julian@gmail.com --- I can confirm that your patch prevents the unmanaged window to spawn.
It turned out that also Wine's system tray icon feature can be problematic (e.g. Battle.Net client), as it (independently of the desktop environment?) spawns a 100% translucent window with on-top property. Is it really necessary that it is always on top? This can make it placed on top of fullscreen windows and thus can/does prevent direct scanout as well. As a result, an exception for 100% translucent windows was added to kwin's direct scanout feature, but this is probably better also fixed at the root?