Hi, I've found the same bug on two different Debian buster systems running kmail 5.9.3, both of them upgraded from stretch, jessie, etc. In both of them a small square, similar to the screenshots provided by the original poster and showing what looks like a piece of some other window (web browser, terminal, etc.), appears at the bottom left of the list of messages. This square behaves as if it was part of the kmail window, moving with it and being repainted if covered by another window. The square seems to have no effect, other than the cosmetic.
Regards, Josep
