I don't understand why this important and many users concerned bug (easy to solve) exists since so many years and all the discussions refer to mark something as duplicate and outdated. This bug https://gitlab.gnome.org/GNOME/gdm/-/issues/372 still exists in my fresh installation of ubuntu desktop 20.04 from yesterday. My tv is the primary screen, notebook via hdmi is open for powerbutton. Login screen appears unreadable for me on the build in notebook screen.
I had to do some additional steps:
If you haven't done it yet, go to Settings → Devices → Displays and set your primary monitor. Save your settings.
Open a terminal and run the following:
sudo cp ~/.config/monitors.xml ~gdm/.config/monitors.xml sudo chown gdm:gdm ~gdm/.config/monitors.xml sudoedit /etc/gdm3/custom.conf
In the file uncomment
WaylandEnable=false, and save.