First one - IntelliJ (which Android IDE is based off) supports scaling nowadays.

Try running

  xrandr --dpi 200

(try it with lower values like 144 if that's too large)

and restart Android IDE.

Second monitor: you need to render the second screen at High DPI and then downscale. Example script (with two monitors):

  xrandr --output eDP-1 --off
  xrandr --output DP-2-1 --left-of eDP-1
  xrandr --output DP-2-1 --right-of DP-2-2
  xrandr --output DP-2-2 --scale 1.5x1.5 --pos 0x0
  xrandr --output DP-2-1 --scale 1.5x1.5 --pos 2880x0

Hi, and thanks for the hints. These were discussed over at arduino.cc a while back, and I had some small measure of success with them, but it was very messy to get this working with the apps that needed them (only). And, ultimately, it broke savagely with problem #2 -- when I move between displays with drastically different DPI's. I don't think that second problem is totally intractable ... but I do think it's a long way of being fixed (on any platform).

EDIT: Oh, and yes, the 'scaling support' feature was added into the version that I was trialling of arduino IDE a while back (I forget the #'s). It was meant to have been resolved, but I noted on the various forums that lots of people (including myself) couldn't get it to play nice. Investigations were on-going whether it was a KDE, xorg/wayland, nvidia, phase of the moon, or some other related problem.

Did you try it? What issues are remaining? Links to the bug tracker reports? Legitimately curious.

> Many applications use toolkits that are unable / unwilling to scale.

Isn't that true on every platform?

Well, except Mac OS. There are ways to "forget" about scaling and render in pixel terms, but chances are your app already looks like shit if this applies to you. The native UI toolkit renders to points which are scaled pixels.

Then there's image/graphic density; I can live with that.

The only thing that can be a problem on OS X, and I do mean ONLY thing that I've run into, is some games don't scale at all. So you'll open a game that draws into its window (I recently experienced this with Shenzhen I/O) and everything in the window is rendered like you had one to one pixel scaling. It works perfectly, everything is really tiny.

If you put it in full screen the monitor gets adjusted and it's not an issue.

At the same time other games don't have the problem. For example I've been playing Stardew Valley and it works perfectly fine in windowed mode looking exactly the way you'd want it to.

But normal applications based on UIKit and not a GL surface or something else like that? It's perfect.

> Isn't that true on every platform?

I'm assuming so.

I've been using KDE as primary environment on laptop & desktop since the late 1990's. At dayjob I have to suck it up and use Win7 - typically as a VM (local or remote) running over RDP or VNC - but occasionally native boot. It really breaks badly with high DPI, but it's quite an old OS. I rarely touch MS Windows 8(.1) but when I did, on this hardware, it was struggling to maintain sanity with scaling. I've avoided playing with MS Windows 10, and OSX -- of course, OSX tends to run on its own hardware, so has a bit of an advantage there. I do have a MBP somewhere, and could try cranking it up on the high-DPI 32" monitor ... but a) I'm happy to defer to the expertise of people who have done this and are more familiar with the platform, and b) it won't help me (as I'm not going to move platforms).