I am using Mint on a laptop.
Sometimes I connect it to a TV and make the TV the only active screen. I tend to forget resetting this and just plug out the cable from the pc, which leaves me with having no screen.
It would be good to have a checkbox in the Display settings window where it would be possible the select the screen that is a laptop screen and is automatically turned back on if the "secondary" screen is not connected anymore.
The checkbox could also be named "Revert to this screen if no other connected"
Setting the checkbox could also make a sign appear in one of the corners of the selected screen to show that this screen has a unique status