It's report #709684. I'm aware I didn't include nearly enough info
in the report, but hoped that the maintainer would give me guidance in
what would be relevant to the problem at hand. I wasn't even really
sure whether I should identify the problem as "gnome" or "gnome-shell".
I did find a workaround on the web from someone who had a
similar problem. If I go to Advanced Settings and adjust the text
scaling factor the problem clears, even if I bring the factor back to
1.0. I have no idea what that indicates, but the problem goes away
until my next reboot if I do that.