FWIW variable DPI has been supported for a long time (try typing `xrandr` with multiple monitors and you'll see their actual dimensions). The issue isn't on the X11/Xorg side but on the application/toolkit side and extending EWMH so that the window manager can request scaling from toplevel windows.

▲

dsr_ a day ago | parent [-]

I'm using three monitors with 2 DPI (4K/42" x2 and one 1920x1200 turned portrait) with XFCE.

When I first connected it, I set a multiplier for the portrait monitor and I haven't thought about it since. It holds a chat system, status bar, notifications and background music.

	▲	thom 21 hours ago \| parent [-]
		Yeah, I'm using a fairly esoteric dual-screen laptop in Xfce which took five minutes of xrandr to get it's 2x mode looking great. Keep expecting to be forced to use Wayland one day, but it's not happened yet.