Intermittent flashing menu bar on top panel

I've been having this problem for a long time now.

Native applications such as Caja and Evolution (not Firefox, not LibreOffice) will very briefly show a menu bar at the top. When I say "briefly", I mean I had to record my screen and go back frame by frame to figure out why these applications would act like they had a case of the hiccups. It does this around every 10 seconds.

I've looked in Tweak, nothing seems to change re decorations and such. And I've looked in Appearance, and nothing in there seems to help either.

Release 20.04.2 LTS (Focal Fossa) 64-bit
Kernel Linux 5.4.0-66-generic x86_64
MATE 1.24.0

Any help would be appreciated.

I apologize. Apparently I had created a duplicate post at some point and accidentally bumped that one.

I had solved this problem, but didn't write down the solution, so naturally forgot what it was.
The problem has cropped up again and is quite annoying. So here's the video again on a different Peertube instance because is about to shut down...

Here's the duplicate post. Evolution Refresh(?) bug - #2 by teslas_moustache

Whatever the admins want (me) to do is fine.