https://bugs.winehq.org/show_bug.cgi?id=45899
--- Comment #12 from Guilherme Silva guih.rox@gmail.com --- I think I've found the culprit. It's the KWin compositor.
I've been trying to reproduce this with some different desktop environments (Cinnamon, MATE, XFCE) and so far I've been only able to reproduce this on KDE. So I just tried disabling the KWin compositor (Alt+Shift+F12), then I opened TFlash again and the issue was gone. It'd be better if somebody else could reproduce it though, just to be 100% sure.