display-settings can't properly handle 2 and 3 display setups.

Submitted by Michael Neuffer

Assigned to Jérôme Guelfucci

Description

Created attachment 6580 Screenshot from display-settings panel when all 3 displays are working correctly.

Unfortunately display settings doesn't switch on display 3 voluntary and even switches off display 1. Getting all three switched on is quite often a longish juggeling act restarting the whole x-server and clicking on "Use this display" over and over. Sometimes the screen then gets switched on for 2-3 seconds and then immediately off again just to repeat the whole mess.

Interestingly enough though, the Display-Lockmanager lightdm never has any problems and starts on all three displays. Only once the xfce desktop itself starts after the login, the display or displays get switched off again and must painstakingly switched on again by the above process.

Sometimes it is enough (luckly most often in the 2 display setup where display 1 is missing) just to lock the screens and to log in again (a few times) to get all displays working properly.

Once all three displays are switched on successfully I run the xrandr command below which then in turn messes up the display-settings display since it can't handle scaled displays, as visible from the attached screenshot (but that is only a trivial issue compared to the main problem)

xrandr --output VGA-0 --off --output DP-5 --off --output DP-6 --mode 3840x2160 --pos 6080x0 --rotate normal --output DP-4 --mode 3200x1800 --pos 0x0 --rotate normal --output DP-3 --scale 1.5x1.5 --mode 1920x1200 --pos 3200x0 --rotate normal --output DP-2 --off --output DP-1 --off --output DP-0 --off

The three displays are -left to right- a 15,6" laptop display, a 24" FullHD display (scaled 1.5x1.5) <- This is missing in the 2 display setup and a 28" UHD display

Attachment 6580, "Screenshot from display-settings panel when all 3 displays are working correctly.":

Version: 4.12.0