Created attachment 144230 [details]
confused display configuration

SUMMARY

My Laptop monitor 14"  2560x1440 looks best at 175% scale
My external 4k 27" monitor looks best at 150%

For whatever reason 'Display Configuration'  remembers/shows 3840/1.75=2194 x 2160/1.75=1234 in the visualisation of the displays as well as for the relative positioning (see attachment buggy-setup.jpg).  Tested with 'extend to the right' and with 'extend to the top'.   Resolution is properly displayed as 4k.

I've created a new user and logged in and everything works as expected.  System settings show 4k in the visual represtation and positioning external monitor to right and on the top  results in properly aligned displays.

STEPS TO REPRODUCE
1.  open 'Display Configuration'
2. *check that/when* the size in the visualition is not the real resolution
3. create an 'extend to the right' setup, 'Apply'  

OBSERVED RESULT
only 2194 instead of 3860 pixel of the external monitor are used  (see also attachment buggy-layout.jpg):

EXPECTED RESULT
full 4k size of the external monitor is used

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.23
KDE Plasma Version: 5.23.3
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.3
Kernel Version: 5.14.0-1008-oem (64-bit)
Graphics Platform: X11
Processors: 8 × Intel® Core™ i7-8565U CPU @ 1.80GHz
Memory: 15.3 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 620

ADDITIONAL INFORMATION

1) Xrandr output of buggy-layout:
 ✘ ach@it01lt  ~  xrandr | grep -C2 '^[a-Z]'
Screen 0: minimum 320 x 200, current 4754 x 2160, maximum 16384 x 16384
eDP-1 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
   2560x1440     60.00 +  59.99    59.99    59.96    59.95* 
   1920x1440     60.00  
--
   360x202       59.51    59.13  
   320x180       59.84    59.32  
DP-1 connected 3840x2160+2560+0 (normal left inverted right x axis y axis) 697mm x 392mm
   3840x2160     60.00*+  29.98  
   1920x2160     59.99  
--
   640x480       75.00    60.00    59.94  
   720x400       70.08  
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)

2) With only external 4k monitor: during early login, before plasma background is shown,  only the left 2194 pixels (estimate) instead of the full 3860 are draws.   When plasma started redraw displays the full 4k resolution