Even Win10 struggles when you move windows between different DPI domains. Apps will slide in HUGE or tiny until you get past the midway point. And when the system goes to sleep everything can go to hell. You can come back to small message windows being blown up to huge sizes or windows crushed down to a tiny square. You can forget about laying out your icons perfectly on your desktop too, they'll get rearranged all the time. Even more fun when you remote into a high DPI display with a low DPI display. It actually works pretty well, but stuff will get shrunk or blown up randomly when you to back to the high DPI display.

>Apps will slide in HUGE or tiny until you get past the midway point.

This is a non issue to me. I don't use apps half way across 2 screens. Its just a minor bit of weirdness while dragging them.

Logically having your OS maintain a consistent UI size makes sense until you try it without.

I'm running a couple medium high density monitors alongside one of the highest density ones available. I don't scale the HIDPI monitor at all, which means when I drag windows to it they are tiny. Instead it works in two ways, as a status screen for activity monitors/etc and as a text/document editing screen. AKA putting adobe acrobat, firefox or sublime/emacs/etc on the high DPI screen and then zooming in gives all the font smoothing/etc advantages of high DPI without needing OS support.

So the TLDR is, turn off dpi scaling, and leave the hidpi screen as a dedicated text editor/etc with the font sizes bumped to a comfortable size. Bonus here is that the additional effort of clicking the menu/etc will encourage learning keyboard shortcuts.

Check out user.js, it's a built-in feature. You can declare which config values you want under Firefox profile folder.