High-DPI displays and Linux

"Making applications work properly in an environment where displays can have widely varying densities — even on the same system — is not an easy problem to solve."

It's really not. The only challenge is agreeing on a system, which unfortunately is perhaps the area the free software community has the most trouble with.

"A fast rate of progress is arguably not surprising; after all, desktop developers hardly seem like a crowd that would be resistant to the allure of a beautiful new screen. So we can probably count on those developers to fix up the remaining problems in relatively short order."

Desktop developers are also the type who often stick to one system, and apparently we have at least 4 different approaches to solving this problem so far. The easy fixes have already been done. I'm not optimistic that the situation is going to significantly improve any time soon. It's not like the KDE developers are going to wake up and switch to Gnome's scheme when they have some free time next week. This isn't a technical problem that can be solved by a couple hours of debugging.

Apple has solved this problem because they can just declare the entire system by fiat. Microsoft is close behind, because they control most of the software and a little of the hardware. Free software does great when there's a BDFL like Linus, Guido, Matz, or Larry, but the desktop GUI has no such person. The closest we have is Ubuntu, and they're the only ones I think have any shot at fixing this in the next 5-10 years -- and they've chosen a 5th option, "Invent a new display server which is neither X11 or Wayland".