No, you can't have a window with a different pixel size on one screen and another. Trying to emulate this using two windows would not achieve a smooth experience.

OTOH it is questionable if this is really all that important. Most of the time if a window spans more than one screen it's temporary because you are just moving the window from one screen to another.

if you have to do it in the client, aka the application, then it's not transparent to the application

If we can require applications to draw their own window controls, I think we can ask them to handle their pixel densities.

that's a good point. window controls should be at the proper scale too.

It's not transparent on Wayland or Windows (no idea about mac) either if you want actually properly drawn stuff.

Well, ok, on Windows if you keep to certain standard elements of Windows API and only use standard widgets you could get close to transparency.

It'll not be transparent to an application using basic X APIs, but nothing prevents a higher level UI toolkit from implementing it properly.