Earlier quoted context omitted.
Separate scaling fractions on separate monitors doesn't work under X. Well, I lie: it does work under zaphod mode, but no applications other than Emacs support that.
Heh. Just today I started fooling around with a new X11 setup on a barebones Ubuntu Server VM with just xorg, xinit, xterm, Emacs and i3. It's pretty neat learning about iommu groups and doing NVMe passthrough with KVM/Qemu, and also messing around with the new (to me) Spice/virgl 3D acceleration. I was impressed I was able to play YT videos in the Ubuntu Virtual Machine Manager with hand-built mpv/ffmpeg + yt-dlp se…
Then, gradually, these things disappeared from Linux, for no good reason; you can still configure them but someone decided in their infinite wisdom that some of the most compelling features just weren't really needed anymore, in favour of rewriting the XDM again and again until now there's too many of them and none of them are really any better than what we had in the 90s.