I use three monitors, one of which is in a vertical position. Sometimes the login screen on Pop OS appears on the main monitor, sometimes it appears on a vertical monitor, but in a horizontal position. Like it or not, I have to log in with my head tilted.
Although if you look at the positive side, this can be counted as stretching in the morning. But I don’t need it and it’s very annoying. Believe it or not, this has been going on for almost 2 years and every time I log in I feel like I need to fix it, but after logging in, I forget and get lost in my works.
Eventually, I got fed up with this and I decided to dedicate 10 minutes of my time to figuring out how. It turns out that not even 1 minute spent, it’s really easy to fix it.
Once you log in, make sure your primary display is already set in your main monitor. Go to your Settings > Display and set your Primary Display. The next login screen will be shown at your primary display.
After the panel is already on the main monitor. Run this command on your terminal, this will change the login screen to be always shown on primary display:
sudo cp ~/.config/monitors.xml ~gdm/.config/
Voila! Now try to reboot and log in normally on your main monitor.