This is pretty cool - especially that it's at the point where it can be used with a real window manager. I'm curious why multiple screens is considered legacy baggage and thus out of scope, given how common multiple monitor setups are these days. I also have zero familiarity with X internals, so don't know if multiple monitor support is a horror show that'd be miserable to support.
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.