How to display GDM on secondary screen with Cinnamon
I recently started using for the first time a laptop running Linux with an external screen. Unfortunately, out of the box, GDM is only the laptop built-in monitor and not the external screen.
If you look this issue online, you'll find plenty of resources like this or that which boil down to configure your screens in your desktop environment, and then copy ~/.config/monitors.xml to ~gdm/.config
WHat they do not tell you is that this will only work if the desktop environment and the GDM use the same rendering server (Xorg / Wayland) ! There is even a 9 years old bug in GDM for it !
So as a Cinnamon user, which uses Xorg as it doesn't support (yet) Wayland, any setup I would do wouldn't have any effect. I had to log in with Gnome 3, do my setup, copy the file, and the switch back to Cinnamon.