xserver and splitting ultra-wide monitors

[Michael Wyraz]

Hello Xorg developers,

about a year ago I investigated in how to split ultra-wide monitors on 
linux into multiple virtual monitors. While this is basically possible 
with xserver and xrandr, a combination of different issues stops from 
using this feature on most desktops. Unfortunately, the issues still 
exist. I'd like to start another attempt to get this finally work.

The first step would be xserver and xrandr.


It is easily possible to split a monitor with xrandr into two:

   xrandr --setmonitor VIRTUAL-LEFT 2560/0x1440/1+0+0 DP-4
   xrandr --setmonitor VIRTUAL-RIGHT 2560/1x1440/1+2560+0 none

For the second monitor, the output must be set to "none" which is 
obviously wrong since it is connected to a device. The reason why it is 
set to "none" is some code in xserver that removes an monitor if another 
one is added to the same output:

https://gitlab.freedesktop.org/mwyraz/xserver/-/merge_requests/1/diffs

I believe, this code should be removed entirely for two reasons. The 
first is, that two virtual monitors on one output is perfectly valid. 
The other is, that the function is about adding monitor. It should not 
be silently delete another monitor.

If there is a reason to delete one monitor if another is added, such 
functionality should be part of the tooling (like in xrandr) - this 
would also allow to implement different behaviours (e.g. a warning or an 
option to enforce to have multiple monitors on one output).

What do you think about this? Could you help to get this solved?

If so and this could be fixed, my next step would be to address the 
related issue in gtk (https://gitlab.gnome.org/GNOME/gtk/-/issues/2013) 
so that gtk based desktops would properly work on a splitted ultra-wide 
screen.

Kind regards,

Michael.