External display displays "ghosted" afterimage of minimized/closed application

I just finished using an external display, not least because I wasn’t able to type – after minimizing Firefox Nightly, a “ghost” (uninteractible) afterimage remained, covering half of the display.

This seems like an OS problem since the Window Manager should be responsible for ensuring that overlays are destroyed, since Firefox shouldn’t manually be displaying itself as an overlay. However, yes, I’ll also report this at bugzilla.mozilla.org, just tomorrow.

Has anyone else experienced this?

I couldn’t screenshot it because I don’t know how to capture a screenshot of multiple monitors via Android, and I just couldn’t capture a useful photograph of it.