[pengaru]

The way multihead works in modern XOrg is basically a hack to enable seamless dragging of windows across physical display boundaries.

Once upon a time you'd do multihead on X with discrete screens and your display environment variable would be something like DISPLAY=:0.0 vs. DISPLAY=:0.1 where the last digit after the dot was the screen number. But your X client would then be confined to that screen. In this old manner you could probably have per-screen DPI stuff work somewhat, but you wouldn't be moving windows across physical monitors like we take for granted today.

Having your X clients connect to DISPLAY=:0 and somewhere behind the scenes that X server is putting its windows across physical displays or moving from one to the other seamlessly is basically Magic (look up XINERAMA) that the protocol is largely ignorant of, so the DPI differences among those physical displays are pretty invisible to the random X client.

[YarickR2]

IDK if ability to drag a window between screens is something more important than properly supporting different DPIs; but that might be a decent level of ignorance on my part

[pengaru]

Eh, I lived with the limitation back in the XFree86 4.0 / beta multihead support days on an array of Matrox Millenium PCI cards.

While it technically works, you quickly discover the importance of being able to organize windows to different physical displays after the programs are already running / the windows have been created.

Requiring exiting and relaunching things to migrate them to different physical screens gets old fast.