I have a Librem 15v4 and I often work with an external monitor.
Plugging the monitor in via HDMI generally works fine. But some large fraction of the time (maybe 70%), unplugging the HDMI doesn’t cause PureOS to recognize that monitor has been unplugged. The computer continues acting as if it has an external monitor plugged in. I have to go to Displays and set it to Single Display to get PureOS to put all of the windows on to my laptop screen. Otherwise I have this phantom screen on which I can’t see anything!
Has anyone else encountered this issue on Debian-based systems?
i’d suggest that you try with PureOS 10 Byzantium (devel) GNOME live .iso from the web-site … i’ve recently had TWO external monitors connected (at the SAME time) in all modes (one after the other for testing purposes) from my Librem-Mini-v1 and they worked fine …
the first one - ASUS 24 inch PA-v1 - connected through HDMI to HDMI ( 1920x1200 @ ~60hz )
the second one - ASUS 27 inch TUF-Gaming - connected through DP to DP ( 2560x1440 @ ~165 hz )
newer sw should help when using free-software especially on the graphics side with GNOME.