Qt 5 and 6 disagree on the interpretation of HighDPI scaling environment variables

Environment:

• Linux/X11
• Font DPI forced to 240 via X command-line (-dpi 240 and the X resources database (Xft.dpi: 240)
• two displays, both HighDPI (see qtdiag.txt)
• starting with a clear environment

With this environment, it looks like Qt 5 concluded that a scaling factor of 3 was required, because the fonts and icons look much bigger:

(background is KWrite using Qt 5.15 with KDE Qt Patch Collection, foreground is Qt Linguist, from Qt 6 dev branch at df2e07549e6edd1d271cce1d86cadd4a33dfd8fc).

So I usually force QT_SCREEN_SCALE_FACTORS to the value "2;2".

In this scenario, they look to have the same font and icons sizes, but the fonts are too small. Worse, Qt Creator (not shown, see QTCREATORBUG-29803) is even smaller, making it unusable.

If I set QT_AUTO_SCREEN_SCALE_FACTOR to 0:

It looks like the Qt 5 application changed but the Qt 6 one did not. Considering the majority of my desktop is still Qt 5, that's the set up I have optimised for. It used to work for Qt Creator (for some reason) but is now broken.

QT_FONT_DPI has no effect and appears to never override anything.