[davidp]

Nothing built on X does mixed DPI well, because X itself (in most configurations) treats multiple monitors as simple viewports onto a single screen. A single screen with multiple DPIs doesn't make sense virtually any more than it does physically.

Wayland is supposed to help with this, since it has a more modern take on screen virtualization (a la Windows or Mac). Getting all the user software to work on Wayland (via XWayland) is a nontrivial ongoing task. I'm not sure what Gnome Shell's state of affairs is w.r.t. Wayland, but I do know the ecosystem missed the boat with Ubuntu 18.04, which elected not to make Wayland the default due to stability and usability issues. So it might be a while before a major stable distro makes the move.