Bug#921323: gdm3 ignores external monitors

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Package: gdm3
Version: 3.30.2-1
Severity: important

Device: Thinkpad T470

Debian Version: Buster with latest updates

gdm3 ignores file /var/lib/gdm3/.config/monitors.xml.

This leads to a weird situation where the laptop is in the
dockingstation with lid closed and two or one external monitors
connected but the login screen is not visible after boot or reboot.
Therefore, the laptop goes to suspend because lid is closed and no log
in screen is outputed on the external monitors.

Interestingly grub is displayed on the external monitors but as soon as
gdm3 kicks in the screens go blank and the laptop supsends. So its not
a general problem, because once booted with lid open and all settings
made in GUI and then lid close the laptop is properly working, just
gdm3 has problems.

After reading https://manpages.debian.org/testing/gdm3/gdm3.8.en.html
/var/lib/gdm3 seems to be the correct destination and also other wikis
point to log in in a Wayland GNOME-Shell session, create via GUI a
~/.config/monitors.xml and then copy this file over to
/var/lib/gdm3/.config/ so that gdm3 is handling the setup the same way.
Because both are running in Wayland this should be fine.

But in my case the monitors.xml is ignored and I am not able to login
or boot properly when laptop is in Thinkpad Pro Dock or Thinkpad Ultra
Dock.

Thinkpad and both docks have all firmware updates installed.