Logout dialog stacks when the sleep button is held.

Prerequisite: Xfce 4.20 (Debian). In Power Manager, set When sleep button is pressed to Ask.

TL;DR: hold down the sleep button of your keyboard to see the effect.

Full: the sleep button of an old Logitech Cordless iTouch Keyboard exhibits an unusual behavior when pressed. The screen fades to black as multiple logout dialogs stack on top of one another. Pressing the Cancel button that many times returns me to my session.

I looked into xev and compared it with a modern wireless keyboard. The difference is difficult for me to explain. However, I then tried holding the modern keyboard's sleep button and got a similar--if not worse--effect.

I suppose there should be a mechanism that locks the logout dialog to a single instance only.