I have a suggestion, which is to coordinate with freedesktop and/or others for a form factor query for apps which allows querying a "main usage type" for the device (like, e.g. phone-like touch-first small mobile device, or laptop-like device, or ...) as described here: https://gitlab.freedesktop.org/xdg/xdg-specs/-/issues/56

I think this standard should ideally live in the desktop environment (e.g. phosh/phoc), since the knowledge which device type to cater for is most likely present at that level, including for corner cases like knowing whether a phone has an external screen plugged in and should be switched to a desktop-like mode.

Such a standard would allow application devs using more unusual frameworks (e.g. raw OpenGL/X11, SDL2, ...) to incorporate their own code to adapt the UI to be more phone friendly beyond just being responsive to resizing. It might also make it easier for your own GTK+/GNOME app patches to runtime hide things based on this query in a toggleable way, rather than separate phone forks that have permanently unconditionally reduced UI.

I apologize if this already exists and/or is already being tracked somewhere else, but I couldn't find anything! I will happily close this ticket and I apologize for any ticket spam if that is the case.