I have also reproduced the problem with the upstream build for 91.0, downloaded from thunderbird.net, in a fully up-to-date Ubuntu 21.04 VM, executed with MOZ_ENABLE_WAYLAND=1.
gfunk, I'd be very interested in knowing whether that's an Ubuntu-specific problem, or can also be observed on Fedora.
I have also reproduced the problem with the upstream build for 91.0, downloaded from thunderbird.net, in a fully up-to-date Ubuntu 21.04 VM, executed with MOZ_ENABLE_ WAYLAND= 1.
gfunk, I'd be very interested in knowing whether that's an Ubuntu-specific problem, or can also be observed on Fedora.