The funny thing is that X11 can actually do heterogeneous dpi and Wayland can't.
Unfortunately you will never find yourself in a situation to actually use a mixed dpi X11 setup (you lose your homogeneous desktop) and Wayland is better at spoofing it (for whatever reason fractional scaling works better in Wayland).
"If you think this idea is a bit stupid, shed a tear for the future of the display servers: this same mechanism is essentially how Wayland compositors —Wayland being the purported future replacement for X— cope with mixed-DPI setups."
Yeah I've done that, I used my linux box for years with a 24" 1920x1200 screen and a 32" 4k screen next to each other.
Doing some basic mathematics and xrandr command-line wizardry to apply scaling factors to each display, I was able to get Xorg to render to a virtual framebuffer and treat the monitors as appropriately scaled windows onto it, so that dragging applications from one screen to the other didn't result in any noticeable change in size.
Unfortunately you will never find yourself in a situation to actually use a mixed dpi X11 setup (you lose your homogeneous desktop) and Wayland is better at spoofing it (for whatever reason fractional scaling works better in Wayland).
http://wok.oblomov.eu/tecnologia/mixed-dpi-x11/
My favorite quote from that writeup.
"If you think this idea is a bit stupid, shed a tear for the future of the display servers: this same mechanism is essentially how Wayland compositors —Wayland being the purported future replacement for X— cope with mixed-DPI setups."