I am unsure where to clone the issue.
For obvious reasons, a client application like thunderbird should not be
able to break the xserver in a way that thunderbird does.
While extending the apparmor permissions makes thunderbird work again
(and thus solves this bug), there is an underlying issue with the
xserver most likely.
When I try to use thunderbird with apparmor enabled, the xserver stops
drawing except for the mouse cursor; and even killing the thunderbird
process does not fix the xserver. That should be prevented on the
xserver side.
Regards,
Erich
