Endless "cycling" of menu item selection

[^rooker]

On a fresh install of Xubuntu 16.04, I've noticed the following issue:

When I press a "special" key on the keyboard, like "XF86Homepage" while having a Qt-based application running and focused, I'm unable to use any (context-)menu of that application anymore:
It's like one would keep pressing the "down" arrow key whenever I open a menu.

Sometimes even random options get selected after a few seconds (unless I press "Esc" to exit the menu fast enough).

First I thought it was an issue with "Clementine" music player, but just now, I've had the same issue with "vokoscreen". That's why I assume it's an issue with Qt :(

Thank you very much in advance.