Is there a reason plasma still disables $QT_AUTO_SCREEN_SCALE_FACTOR?

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1

The environment variable QT_AUTO_SCREEN_SCALE_FACTOR gets explicitly disabled during startup of plasma:

void setupPlasmaEnvironment()
{
    // Manually disable auto scaling because we are scaling above
    // otherwise apps that manually opt in for high DPI get auto scaled by the developer AND manually scaled by us
    qputenv("QT_AUTO_SCREEN_SCALE_FACTOR", "0");

This variable however prevents proper scaling of Qt 5 apps in an X11 plasma 6 session. If I start these apps with the environment variable set, they scale properly. Qt 6 apps are not affected by this variable, as it is deprecated by QT_ENABLE_HIGHDPI_SCALING

I use 175 % scaling. Setting the variable manually in .config/plasma-workspace/env/high-dpi.sh, the variable gets overwritten by startplasma.

Examples:

Qt 5 with scaling

grafik
grafik1532×1735 306 KB

Qt 5 with only font scaling

grafik
grafik1409×1323 178 KB

2

I put these variables in startkde/startplasma.cpp, so far with no bad side effects:

qputenv("QT_AUTO_SCREEN_SCALE_FACTOR", "1");
qputenv("QT_ENABLE_HIGHDPI_SCALING", "1");

The last one is a bit superfluous, as Qt 6 apps have highDPI scaling enabled by default. I have no adverse effects in X11 or Wayland. Tested on screens with 100 %, 125 %, 170 % and 175 %. I’m even using different scale per monitor, something highly unsupported by KDE on X11!

I suspect it works now, because Qt 6 is way better in terms of scaling for different pixel densities than Qt 5. Qt 5 got a lot better during it’s lifetime, all the different hacks in KDE, that were still needed for e.g. Qt 5.6, are no longer needed in Qt 5.15.

3

This is annoying for me because I use QT_AUTO_SCREEN_SCALE_FACTOR to make the Kiwix AppImage GUI readable. The problem is KDE-Plasma session save/autostart. When I login and Kiwix gets auto started it is tiny. No matter where I try to set QT_AUTO_SCREEN_SCALE_FACTOR=1 it doesn’t stick. I tried in the .desktop file, in user profile and env scripts, systemwide, etc… :frowning: