I've been running X11 on retina displays since 2014 without much of an issue. The only big offenders were chrome and firefox. Nowadays everything just works.
Obviously the web browsers are a big requirement.
...but I've had trouble with the defaults for almost everything I've tried: xterm, libreoffice, browsers. Under KDE the menus are too small by default. Under non-Gnome-or-KDE window managers or desktop environments almost everything is rendered too small.
The chances are that I can't just install an application with `apt-get` and have it do something sensible by default without a lot of fiddling and customisation.
Xft.dpi: 144
I really appreciate you taking the time.
Unfortunatelyh X.Org is based on the lowest-common denominator code, and nobody dared to make a redesign that would really split from the X.Org mission of "providing a code dump for vendors to start working"
> or a high-dpi laptop with a low-dpi external screen
Yep, that's where I am right now, which is why I wanted to mention the third option - our IT support people upgrading everyone's laptops didn't even know this could be done.
Much of this criticism of X was prescient for things to come. Autotools, dependency problems, modularity issues, compatibility, "DLL-hell", etc. Many of these problems don't have solutions today that are orders of magnitude better than then.
On the other hand, one thing that's really holding X back, especially on handhelds, is 'Myth: X is "Device Independent"'.
If the difference between a 75dpi screen and a 100dpi workstation screen was too much, today's "retina" displays really struggle to run a usable X. You're heavily reliant on the toolkits (Qt, GTK, etc) to scale things for you and they mostly understand about font scaling but not about widget scaling.
Lots of distributions that might want to use it, for example Maemo Leste, Ubuntu Touch or Gemian, are having to do a lot of UI work and hand-pick applications for "porting" to the screens and input devices (touch) that they want to use them on.
This means that Linux-on-the-smartphone is not yet in any position to give Android or iOS a run for it's money because it really struggles to effectively leverage its existing app ecosystem.
Even ignoring security models, X struggles to provide even a compelling visual of a vision for handheld devices.
Don't even mention Wayland: that has other economic incentives that are not yet properly aligned for widespread success (including X's app ecosystem ones).