I've also been having the same issue for quite some time. Today I've noticed that I'm also having it on another computer *and* I've noticed that it seems to be correlated with having rendering issues with kicad ( https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=838631 ).
Like Antoni Marcinek uninstalling and reinstalling did not help. I did try to reproduce it on a clean virtual machine, but in that case I wasn't able to see it. Both computers that have this issue are liberated thinkpad x200 with an intel graphic card. I could probably try a fresh install, but I would need to find a spare hard disk (and free time to actually do it), so it will take a few days before I'm able to do so. -- Elena ``of Valhalla''
