Created attachment 871365 [details]
Scaling is set to 100% and it's after a reboot, so entire plasma has been restarted

Operating System: openSUSE Tumbleweed 20231213
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.112.0
Qt Version: 5.15.11
Kernel Version: 6.6.6-1-default (64-bit)
Graphics Platform: X11
Processors: 12 × AMD Ryzen 5 5625U with Radeon Graphics
Memory: 13,5 GiB of RAM
Graphics Processor: AMD Radeon Graphics

Issue:
KDE scaling setting in System Settings > Display and Monitor > Display Configuration is only respected by gtk apps, plasma desktop itself isn't scaled (see attached screenshot). Setting force font dpi in System Settings > Appearance > Fonts seems to properly scale everything. Right now when I have screen scaling at 125%, force font dpi is automatically set to 120dpi. But when I try to go back to 100% from 125%, then force font dpi is set to unchecked. It's worth noting that gtk apps seem to scale correctly when force font dpi is unchecked and display scaling is set to 100%. Just the whole plasma interface stays at previous force font dpi setting.

Steps to reproduce:
1. System Settings > Display and Monitor > Display Configuration - set scaling to something bigger than 100%
2. Log out and back in to restart the whole plasma desktop
3. System Settings > Display and Monitor > Display Configuration - set scaling back to 100%
4. Log out and back in to restart the whole plasma desktop
5. System Settings > Appearance > Fonts - confirm that force font dpi is unchecked

Observed behavior:
Plasma desktop is not scaled at 100%, but apps like firefox and qalculate are scaled according to the screen scaling setting.

Expected behavior:
Everything is scaled according to the display scaling setting.

Workaround:
If scaling up from 100% to 125% sets force font dpi to 120dpi and everything is scaled correctly, then setting force font dpi to 96dpi with 100% scaling should also work. And for now this is the workaround that I found (set force font dpi to 96dpi).

Side note:
It seems that force font dpi is the main respected setting for scaling, as leaving scaling at 100% and setting force font dpi to 120dpi results in the same interface size as screen scaling 125% (125% automatically sets force font dpi to 120dpi, so these settings seem closely related).