This is a known and reported bug with gdm3. The current workaround appears to be:
- Go into
Settings > Devices > Displaysand configure your monitors the way you want for your login screen (in your case, internal laptop display disabled). Click the “Save” button when done.
- Copy your user’s
monitors.xmlfile into the home folder for
To copy the
monitors.xml file, open a terminal and perform the following:
sudo cp ~/.config/monitors.xml ~gdm/.config/monitors.xml sudo chown gdm:gdm ~gdm/.config/monitors.xml
Then, reboot and see if your changes persist. If this doesn’t work, try going through the whole process again. It took me two times in order to get it to stick. I was also able to reboot with my external display disconnected and it switched back to my internal laptop display. I rebooted again and reconnected it and it switches back to my external monitor.
Hope this helps!