QT applications and plasma have their icons under-scaled on a HiDPI display when running KDE

[poscat0x04]

Describe the bug
QT applications and plasma will have their icons under-scaled if you only set services.xserver.dpi.
While you can adjust the icon size in the system settings, it does not work for some applications such as Krita.

After some debugging, I found out that this is caused by startplasma-x11 which always sets QT_AUTO_SCREEN_SCALE_FACTOR to 0 even if you've already set them in /etc/profile (via environment.sessionVariables). If I set QT_AUTO_SCREEN_SCALE_FACTOR to 1 (or just unset it) for any QT application then it will display icons with the right size. After some more research, I found this similar issue #96055. According to this comment under that issue, there was a time where QT_AUTO_SCREEN_SCALE_FACTOR is unset but now it is always set (to 1). I think this is what caused this bug and I wonder what might have changed between then and now.

Of course, this issue can be avoided if we use only KDE to configure HiDPI (through display configurations > global scale), that is to not set services.xserver.dpi. But this workaround is not deterministic and does not work when you want to switch between a DE and a pure WM without rebuilding (the latter requires services.xserver.dpi to be set to work properly on a HiDPI screen).

To Reproduce
Steps to reproduce the behavior:

1. use a HiDPI screen (mine is 4k 14")
2. install NixOS with plasma as the DE
3. set services.xserver.dpi to a proper value and set the environment variable PLASMA_USE_QT_SCALING to 1 via environment.sessionVariables
4. open any qt applications that have icons, some examples include

Expected behavior
HiDPI should work on KDE/plasma

Screenshots
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Additional context
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Notify maintainers
@petabyteboy @worldofpeace @ttuegel @NixOS/freedesktop @NixOS/qt-kde

Metadata

• system: "x86_64-linux"
• host os: Linux 5.12.12, NixOS, 21.11.20210704.20887e4 (Porcupine)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.4pre20210601_5985b8b
• channels(root): ""
• nixpkgs: /nix/store/rplqpacaa11i2wlpjbjz71g8vfynnvb3-source

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
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[stale]

I marked this as stale due to inactivity. → More info