On Linux (Arch Linux KDE X11 with global fractional scaling enabled), calling QGuiApplication::setHighDpiScaleFactorRoundingPolicy() with a rounding mode fails to change the actual DPI rounding policy. QImage/QPixmap is still painted with non-uniform pixel sizes, and QWidget::devicePixelRatio() is still a non-integer. Fetching the policy using QGuiApplication::highDpiScaleFactorRoundingPolicy() still returns the rounding mode you set, but it doesn't change the app's behavior.

On Windows, this function works as expected.

Test app at https://github.com/nyanpasu64/qt6-dpi-fail/tree/device-pixel-ratio (note, not master branch). It calls QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor) and clicking the button prints the current rounding policy and devicePixelRatio. On Windows it's "RoundPreferFloor -> 1", and on Linux it's "RoundPreferFloor -> 1.25".

EDIT: This issue occurs on any environment where the scale factor is set through QScreen's "_q_scaleFactor" property or the QT_SCREEN_SCALE_FACTORS environment variable, and not on environments where the platform plugin supplies the scale factor.