Thank you. Having used macOS for years now, where the transition to High-DPI displays was pretty much un-noticable, I was shocked to install Ubuntu on a machine with a High-DPI display and not see everything working right. Mouse cursor too small. Fonts too small. Icons too small. Change this. Now text is OK. Tweak that. Now icons are all right, but it didn't affect Firefox for some reason. Change that other thing. Now mouse cursor is the right size for a while but the system changes it back every time I come back from sleep.

I still haven't gotten it all to work reliably. And I have no idea if it's Ubuntu, if it's the window manager, if it's the desktop environment, if it's X (or Mir, or whatever they are calling the window system these days) if it's individual applications, or a combination of some of those. It's crazy land.