My mini-PC has 3 video ports:
1 - HDMI
2 - VGA
3 - DP
At the time of the screen shot is taken, Monitor 1 & Monitor 3 are powered off, but video cable connected.
However, I still can move windows to Monitor 1 / Monitor 3 - so they are logically “on” to Gnome.
How to prevent Gnome from “using” those “OFF” monitors?
… you could use
<super p> and switch to mirrored mode while they are off.
I do have issues where I have to re-setup my monitors after swapping between mirrored and joined.
When I am using the computer, I can tell Gnome not to use the monitor by toggle the sliding switch of the respective monitors.
When my family members are using it, I want they can start / stop “use” the monitor by powering them on / off .