Some applications have splash screens or update screens that will close automatically after they finish loading or updating their respective software (such as Krita, LibreOffice, Steam, Discord, etc..), sometimes I'll move these windows out of the way while working on some task, and as result these windows will sometimes close while they're being moved.
I've found that while the "Translucency" Desktop Effect is active, this will cause a "ghost" duplicate of the window being moved to appear. It cannot be interacted with and will only disappear if compositing is toggled off. It will also be drawn over if another application takes focus and is brought to the top of the desktop, but won't disappear.
STEPS TO REPRODUCE
1. Open an application with a splash screen or auto-update start window
2. Move that window
3. Hold it in the moving state until it closes
The window will disappear for a few frames and be replaced with a ghost window
The window should disappear without any artifacts left behind
Linux: openSUSE Tumbleweed
(available in About System)
KDE Plasma Version: 5.26.5
KDE Frameworks Version: 5.101.0
Qt Version: 5.15.8
I initially suspected this to be a Wayland issue, as I had not experienced it while using X11, and I made the switch around three months ago. However, when logging into an X11 session I observed the same behaviour, but on X11 I could actually suspend the compositing to find that these ghost windows wouldn't be drawn with compositing off. That's what led me to narrow down that it was actually an issue with the "Translucency" Desktop Effect specifically.
*** This bug has been marked as a duplicate of bug 441379 ***
*** This bug has been marked as a duplicate of bug 439332 ***