How to make KDE use a screen from xrandr 1.5?

Question

I am trying to set up a 5k monitor with Ubuntu. It has two DisplayPort inputs because the bandwidth is too large for one cable. xrandr 1.5.0 comes with support for multiple monitors and tiling output. So far I have managed to set up the two DP outputs as one screen:

xrandr --screen 0 --output DisplayPort-0 --mode 2560x2880 --pos 0x0
xrandr --screen 0 --output DisplayPort-1 --mode 2560x2880 --pos 2560x0
xrandr --screen 0 --setmonitor HPz27q 5120/597x2880/336+0+0 DisplayPort-0,DisplayPort-1

However I can't figure out how to make KDE use this screen instead of the two separate monitors. Here is the output of xrandr -q:

xcreen 0: minimum 320 x 200, current 5120 x 2880, maximum 16384 x 16384
DisplayPort-0 connected 2560x2880+0+0 (normal left inverted right x axis y axis) 299mm x 336mm
2560x2880     59.98*+
  ...
640x480       59.98  
DisplayPort-1 connected primary 2560x2880+2560+0 (normal left inverted right x axis y axis) 597mm x 336mm
2560x1440     59.95 +
   ...
2560x2880     59.98* 
HDMI-A-0 disconnected (normal left inverted right x axis y axis)
HDMI-A-1 disconnected (normal left inverted right x axis y axis)
DVI-D-0 disconnected (normal left inverted right x axis y axis)

I have tried DISPLAY=:0.0 exec startkde in the .xsession-file, but this doesn't work. KDE still treats the two cables as two separate screens. Is it possible to present the monitors as one screen to KDE by using the tiling features? I have tried to find the answers to this in the docs/manpages, however there are no manpages for version 1.5 even though this version was launched in 2015.

Answer 1

As of writing this there seems to be no way to stop KDE from discovering the individual panels. There is however a partial workaround.

In System Settings -> Window Management -> KWin Scripts, enable the Video Wall script , and un-check the Apply to setting in in the settings for this script. Now, applications that explicitly request full screen (e.g. video players, full screen mode in web-browsers, etc) will span over the whole monitor. Windows will, however, still maximize to the individual panels and the KDE task-bar will at most cover half of the width of the monitor.

A task-bar that spans the whole monitor can be emulated by creating another task bar, drag it to to the other side of the screen and in the settings of both task managers enable Show only tasks from the current screen.