Weird repaint bug caused by a misplaced popup menu
Firefox, when running using the Wayland backend with more than one monitor, displays popup menus at the wrong location (but that's a Firefox bug).
If I start moving the window while the popup menu is open, it should dismiss the menu - and I think it does - but the menu stays visible on the screen. It looks like a repaint bug. The menu gets overwritten when anything gets painted over it.
The bug is not 100% reproduceable, but fairly easy to reproduce after clicking around. It requires multiple monitors to trigger the Firefox bug.
I'm using mutter version 3.30.2.