I’ve been playing a little bit more with HiDPI in Ubuntu MATE 14.04. Most things work. It would be usable on a HiDPI screen. I have no previous experience with HiDPI so I did this to establish some sort of HiDPI reference for myself.
On Reddit there are complaints about HiDPI support on Linux. I don’t know, I didn’t expect anything so I’m impressed. Simple solutions tend to impress me the most. I think the GTK 2 way of doing it is quite brilliant (and simple). It’s centered around the fonts. If you make your fonts bigger your windows will become bigger and if you increase DPI your fonts and windows will also become bigger. This affects all GTK 2/3 and QT 4/5 applications so there is no toolkit that looks different from a scaling perspective.
Checkboxes and scrollbars don’t scale at all. One solution could be to make different sized themes. I saw that in KDE 3/Trinity. Icons don’t scale unless you scale them yourself in for example Caja.
Below is a screenshot of my 1920x1200 desktop which is set to 160 DPI instead of normal 96. Obviously, this is not how I use it. Caja (GTK2), GtkHash (GTK3) and SMPlayer (QT4) all scale in a similar way.
So the interesting question is if “modern” toolkits can do better than this? Being able to set any DPI and having other toolkits adjust accordingly are two impressive feats. It would be usable on a 4K monitor (say 192 DPI), but it wouldn’t be perfect. Icons, checkboxes and progress/scroll-bars would be 25% of the normal size (unless defined by a special HiDPI theme). Anyone using HiDPI or scaling in any way?