I suspect(with out reading the source to find out) that screens are the traditional X11 screens as opposed to the modern xrandr combined screen.
Traditionally each screen in an X11 setup was it's own separate thing with it's own separate frame buffer. While technically applications could move between screens, this depended on the application caring enough to do so. It had to maintain two(or more) mirrored windows(one per screen) and keep them all aligned. So realistically no application did this.
The modern method of doing multi monitors on X11 involves one large virtual screen with each monitor assigned a section of it. This has downsides, for example; this is where the myth that X11 can't do mixed DPI setups comes from. But it has one huge massive overwhelming upside. The application does not have to be aware that there are multiple screens and multi monitor setups just work.
Just did a quick `xrand -q` to confirm I'm doing multiple DPIs, etc (cuz laptop and external monitor) on a single screen with 0 issues. Unless the physical misalignment of the monitors, which reflects as a vertical jump when moving the mouse pointer across the virtual boundary, can be considered an issue.
The downside is your refresh rate is locked to the slowest monitor.
Can you pls share smt on how to properly do multi dpi in X? It is hard to find and I struggle with it
> So realistically no application did this.
Old versions of GIMP (back when the toolbars etc. were separate windows) used to let you move any of its windows to a different X screen. And by "move" I don't mean drag - there was a menu where you could select the screen to move to.