High-DPI is still a mess on Ubuntu (and Debian, for that matter). Last time I used Ubuntu on a 4k panel I had to manually edit some xorg config files. I'm using Debian+KDE right now and I had to manually make some adjustments (in a UI, at least) and it still randomly gets confused sometimes.

Not true for me. I've used several distros (including Ubuntu) with Gnome on my 4K XPS and the worst I've had to do is go into Gnome settings and click 200% GUI scale. I'm pretty sure Ubuntu set that automatically.

High DPI is fine as long as you have just one display. However, there's no good way to have one high-dpi display and one normal one (for example, a laptop with high dpi screen connected to a standard external monitor).

There is a good way to have mixed-dpi setup: you use Gnome-on-Wayland (for normal users who expect normal desktop) or Sway (for those who want tiling wm).

Mixed DPI is not coming to X11 displays. If you insist on X11, you are going to have bad time.

Not to mention mixed-DPI. Apple is the only vendor who actually handles HiDPI and mixed-DPI environments really well in my opinion.

macOS can scale different parts of an application differently depending on which screen it is on. So if you are in the process of moving an application from one screen to another, it doesn't change size mid-move.

Windows can't do that, and I've even seen applications where all windows belonging to an application use the same DPI (chosen based on which window is in focus), regardless of the DPI of the screen the window itself is on.

So it seems to me the integration of mixed-DPI into window rendering APIs was not well handled by the development team behind its implementation in Windows.

The most common "solution" I see is lowering the resolution of the high-DPI display, but that's not a solution, that's actually not even a workaround, it is literally removing the problem by pretending my screen is not as good as it is.

4K support seems to vary a bit from distro to distro. Some are good, some are lagging.