(UE 4.26.1)
For some reason, when I use the UE4 editor (compiled for Arch Linux), moving between applications causes Unreal to artifact its window views.
One example would be attempting to open a project, moving to the system file manager to look around, then switching back to the engine to find that the parent windows are now ghosts of the file manager window (or any other visual that was present beforehand) that was present just before switching back to the engine.