My laptop has a HDMI cable connected to a second screen.
When restarting my laptop Manjaro forgets wich was the main screen.
How to force Manjaro remember the last used screen or force to identify the second screen as the main screen?
The configuration is saved in
~/.config/monitors.xml - that means is in your home directory. When you disconnect the external monitor it it will reset that file. If the external monitor is what you want to be the main screen, and is always connected, then set it as primary and run this commands from terminal:
sudo cp -v ~/.config/monitors.xml /var/lib/gdm/.config/
sudo chown gdm:gdm /var/lib/gdm/.config/monitors.xml
Reboot your system after that.
By the way, is not Manjaro that forgets.