https://bugs.winehq.org/show_bug.cgi?id=49979
--- Comment #2 from thomas mielket@gmx.de --- Yesterdays clue could have been a misinterpretation. Today while playing around with a KDE desktop, I noticed that it makes a difference whether the app window is normalized or maximized. If maximized, the effect won't occur. I also was able to click 'through' child window's titlebar and activate ribbon menu items. I wasn't able to move the child window at all until I maximized the app window.
This seems to be an issue of how mouse events are captured as keyboard input after pressing the alt key and keyboard shortcuts were no problem.