Thunderbird strange behaviour. Ubuntu 21.04

There's a workaround for this bug by modifying the thunderbird.desktop file in /usr/share/applications, see the description in launchpad https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1932328

Since the combination of Thunderbird and Wayland triggers this bug, the workaround is to run Thunderbird as a X11 application. This uses XWayland which is an interface that allows to run X clients in Wayland.

As a side effect, fractional scaling will lead to blurry results with Thunderbird (as usual with X11).

For your convenience and for all the people googling this bug, I'll repeat the fix here. To use XWayland by default from the Gnome shell, modify /usr/share/applications/thunderbird.desktop and change the line

Exec=thunderbird %u

into

Exec=thunderbird --display=:0 %u