http://bugs.winehq.org/show_bug.cgi?id=22315
--- Comment #4 from Roderick Colenbrander thunderbird2k@gmail.com 2010-04-18 17:04:25 --- Apparently this works using SetLayeredWindowAttributes on Windows and we seem to offer support for this in Wine. sync_window_opacity in winex11.drv explicitly has such code for the systray. That's perhaps a place to look.