I am trying to make my application compatible with the new "high DPI" system introduced by Qt5, and since I don't have an actual 4K display I am using the environment variables QT_SCALE_FACTOR and QT_SCREEN_SCALE_FACTORS for testing purposes. However I'm getting very inconsistent results for font/widget sizes.

I have two 1920x1080 monitors, I will call them A (left, primary) and B (right) for simplicity.

QT_SCALE_FACTOR=2 produces widgets and fonts scaled up by 2x, on both monitors, as expected.

QT_SCREEN_SCALE_FACTORS="A=1;B=2" produces the normal size on monitor A, and everything is scaled up by a factor 2x on monitor B, again as expected.

QT_SCREEN_SCALE_FACTORS="A=2;B=2" produces weird results: although most widgets are scaled up by a factor 2x, the fonts are still the normal size, and certain widgets like check boxes and scroll bars appear strange (normal size but with thick lines). This happens on both monitors.

QT_SCREEN_SCALE_FACTORS="A=2;B=1" is even worse, on monitor A the widgets are (mostly) scaled up by 2x and the fonts are normal size (as in the previous case), however on monitor B the widgets are normal size and the fonts are scaled down by a factor 2.

Is this the expected behavior? Is there a separate variable to set per-monitor font sizes (like Xft font settings) that I'm supposed to use?