Isn't this how ordinary spanning monitors works? It might be slightly awkward with AR goggles since the relative orientation of the displays will be constantly changing as your head moves, and what happens to a window you have half on and half off of the Macbook's screen when you look away? Or do you want to have the application jump between devices, like appearing on your fridge when you go for a drink? With the old X11 protocol and a daemon in the middle this was possible but the use cases were extremely limited and the security issues made it a pain in the ass to actually use. With distros moving away from X11 this is only going to get harder, and you have to ask yourself how much you really want it.

This would mean the goggles would be basically just a dumb display for the Mac. It would also be weird to try to move an AR app onto your Mac.

I feel like multiple people did it back in the original X11 days, and almost certainly when compiz was the new hotness

Nearly 20 years ago there was an OS X screensaver that would capture all the window buffers and then float them around the screen as they rotated various ways. Another app would save a snapshot of all windows as Photoshop layers with appropriate transparency (before blur was added). X11 was always network transparent, though I don’t think windows could easily be moved from one client to another.

Even classic Mac OS, which wasn’t designed to be “rootless” could be run that way could have its windows mixed with OS X windows in the classic days.

As others have pointed out, there is already a PoC app that is doing it, so it seems if Apple wants to, it’s completely in their power. However, does this match their vision? (pun intended) Time will tell.