It's really not "working" per se. VRR was breaking on X11, sandboxing was breaking on X11, fractional scaling and mixed DPI were breaking on X11.
How did we achieve HiDPI on X11? By changing Xft.dpi (breaking old things) or adding random environment variables (terrible UX - do you want to worsen Linux desktop's reputation even more?). Changing XRandR? May your battery life be long lasting.
There's genuinely no good way to mix different DPIs on the same X server, even with only one screen! On Windows and Mac, the old LoDPI applications are scaled up automatically by the compositor, but this just doesn't exist on X11.
I focus on DPI because this is a huge weakness of X11 and there is a foreseeable trend of people using HiDPI monitors more and more, there are tons of other weaknesses, but people tend to sweep them under the rug as being exotic. And please don't call HiDPI setups exotic. For all the jokes we see on the eternal 768p screens that laptop manufacturers like to use, the mainstream laptops are moving onto 1080p. On a 13" screen, shit looks tiny if you don't scale it up by 150%.
You can hate on Wayland, you may work on an alternative called Delaware for all I care, but let's admit that X11 doesn't really work anymore and is not the future of Linux desktop.