I don’t have a consistent reproducible use case. However after a minute or so of interacting with the editor results in the artifacts demonstrated here.
The editor is publicly visible at https://instaballoons.com. Any ideas what could be going on?
Looks rather like a browser bug, possibly in combination with a very funky font. I haven’t seen reports of this one before. But in general, ProseMirror doesn’t render fonts so if font (or image?) rendering goes to pieces, it’s unlikely to be ProseMirror’s fault.