No HiDPI scaling on X11 with AA_EnableHighDpiScaling

Various applications using Qt set AA_EnableHighDpiScaling on startup, with Qt 6, this is set by default. When using those applications on X11 with a HiDPI monitor, they show up unscaled and are too small to be usable.

qt.qpa.screen: adding QXcbScreen(0x55c178bd8830, name="Virtual-1", geometry=3840x2160+0+0, availableGeometry=3840x2133+0+0, devicePixelRatio=1.0, logicalDpi=QPair(96.0,96.1), physicalSize=344.0x194.0mm, screenNumber=0, virtualSize=3840x2160 (3840.0x2160.0mm), orientation=Qt::LandscapeOrientation, depth=24, refreshRate=18.4, root=528, windowManagerName="IceWM 2.1.1 (linux-gnu/x86_64)") (Primary: true )

Setting QT_AUTO_SCREEN_SCALE_FACTOR=1 explicitly makes no difference.

It appears like it's necessary to either override Xft.dpi in the X resources or pass QT_USE_PHYSICAL_DPI=1 now. This was not necessary before Qt 5.14, so this is a regression.

It seems like the code to detect a HiDPI setup based on physical dimensions was removed without replacement?