I have a dual monitor setup. Native resolution is 1920x1080. So the complete screen area is 3840x1080. I can set up this with System Settings in KDE or with xrandr. Works as expected.
For various reasons I want to scale the size of the screen. That is, have a larger frame buffer and then scale the screen. So everything on screen appears smaller.
I use an xrandr command for that:
xrandr --fb 4320x1215 --output HDMI-1 --scale 1.125x1.125 --pos 2160x0 --primary --output HDMI-2 --scale 1.125x1.125
Note the total screen area is now 4320x1215 instead of 3840x1080.
This worked fine in KDE4.
In KDE5 using Plasma it does not work anymore. That is, the screen size IS 4320x1215 now. I can move my mouse through that area. I can move windows into that area. But the Plasma desktop does NOT resize to the new screen size. The task bar is stuck at the old position.
As you can see in the images, X knows the screen is larger now. I can shift windows below the task bar or right of the desktop area. But the desktop does not resize. It used to resize correctly in KDE4.
When I log out of KDE the screen size is set to 1920x1080 again. The two monitors show the same login screen. That is how it was in the past.
I did create a wallpaper with the full screen size 2160x1215, but KDE insists on scaling it to 1920x1080 anyway.