Maybe the system is remembering the external monitor as a configuration where the laptop screen was once turned off. So switching to that configuration involves turning off the laptop screen. To fix that try deleting all memory of the monitor configurations:
rm ~/.config/monitors.xml ** Package changed: xorg (Ubuntu) => mutter (Ubuntu) ** Tags added: multimonitor ** Changed in: mutter (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu-X, which is subscribed to xorg in Ubuntu. https://bugs.launchpad.net/bugs/1903829 Title: laptop screen turns off when I connect an external monitor To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/mutter/+bug/1903829/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~ubuntu-x-swat Post to : ubuntu-x-swat@lists.launchpad.net Unsubscribe : https://launchpad.net/~ubuntu-x-swat More help : https://help.launchpad.net/ListHelp