Comment 5 for bug 1933996

Not sure whether the upstream issue, which is several months old, is the same. The present one seems to have kicked in for several users at the same time at the end of June 2021.

It looks like it is somehow connected to Thunderbird.

Also see this comment, for one way of reproducing it: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1934527/comments/4

I can also confirm that it does not seem to occur if Thunderbird isn't running at all.