> Mixed DPI is basically unusable on Wayland due to X11 apps but last I checked, X doesn't even support this at all.
Xorg/RandR provides the necessary info, but it is up to the apps (really, toolkits) to work with that and to do it properly it needs the cooperation of both window managers and applications(toolkits) to define some messages/events for scaling, etc. It shouldn't be any different than the other common stuff between X clients, like ICCCM and EWMH (if anything, it should be part of a new version of EWMH), but nobody has bothered so far.
Also there is the -separate- issue where under XWayland, Wayland is lying to X about DPI so apps cannot scale themselves if needed. E.g. Lazarus/LCL has its own scaling logic which works even with the Gtk2 backend (Gtk2 has no scaling logic itself) under Xorg but under XWayland the compositor is lying to the program that is running at 96 (or whatever "non-hidpi") DPI and does bitmap scaling instead (so applications appear blurry). Or at least this is how it is under KDE (which is the only DE i have installed on my PC to use as a secondary environment for testing out stuff - i mainly use Window Maker myself as my main environment).
Xorg/RandR provides the necessary info, but it is up to the apps (really, toolkits) to work with that and to do it properly it needs the cooperation of both window managers and applications(toolkits) to define some messages/events for scaling, etc. It shouldn't be any different than the other common stuff between X clients, like ICCCM and EWMH (if anything, it should be part of a new version of EWMH), but nobody has bothered so far.
Also there is the -separate- issue where under XWayland, Wayland is lying to X about DPI so apps cannot scale themselves if needed. E.g. Lazarus/LCL has its own scaling logic which works even with the Gtk2 backend (Gtk2 has no scaling logic itself) under Xorg but under XWayland the compositor is lying to the program that is running at 96 (or whatever "non-hidpi") DPI and does bitmap scaling instead (so applications appear blurry). Or at least this is how it is under KDE (which is the only DE i have installed on my PC to use as a secondary environment for testing out stuff - i mainly use Window Maker myself as my main environment).