I turned on my newly installed Pop OS and then suddenly the login screen is located on the left monitor.
It’s horrible! Because I put my left monitor into a vertical position, and the Pop OS login screen orientation is in horizontal mode. Therefore I had to tilt my head -90 degrees to type my password and log in.
Fortunately, the fix is quite simple. After you log in, make sure you set the primary display to the correct monitor via Setting > Displays.
Once your monitor configuration is correct, you can easily copy that configuration to GDM which handles the login screen:
sudo cp ~/.config/monitors.xml ~gdm/.config/
This will copy the current settings of our multiple monitors to the Gnome Display Manager configuration.
Then reboot to apply changes.
From now on, the primary display will be the screen where the login prompt will show. 😉