I know the implementation needs a lot to be desired but just in case you were not aware, the Oculus Rift shipped with this feature and it still exists via the link on Quest AFAIK. You pick the last icon on the UI bar that appears and it lets you select any window to pop out separately
Here's a screenshot of 4 windows plus the deskop.
https://pasteboard.co/hPKLrKgp5vjy.jpg
This features has been there for nearly 9 years?
(note: In my view there was passthrough and I could see my room but the screenshot function didn't include that).
It seems limited to desktop + 4 popout windows but I know it used to support more because when I first got the Rift I tried to see if I could get 6 videos running + desktop all at once. It worked.
There are issues though
(1) it's just showing the desktop's image, that means it can't adjust the resolution of the window. Ideally you'd pick a devicePixelRatio and a resolution and it would re-render but no desktop OS is designed to do that.
(2) It's Microsoft Windows, not an OS that Facebook controls which means they're at Microsoft's mercy and whatever features Windows supports. No easy way to add better VR controls. For example, IIRC, you can't size a window directly. Instead you need go to the window showing the full desktop, size it there, that will then be reflected in the popout window. If that's not clear, there are 2 size. The size in VR. The resolution of the window. If the window is a tiny 320x240 pixel window on your desktop and you make it 5x3 meters in your virtual space, all Oculus can do is draw those 320x240 pixels. You can go back to your desktop, size the window to 1237x841, now Oculus can draw those 1237x841 to your 5x3 meter VR pane. But, since Oculus doesn't control windows (or maybe they're lazy), there is no way change from 320x240 to 1237x841 by directly manipulating the pane in VR. Instead you need to do that in the desktop pane in VR.
(3) IIRC it's got some weird issues with the active window. Since it's really just grabbing the textures of the Windows OS compositor and showing then in different spaces, that's separate from Windows' itself's concept of which window is the foreground window. So, as you select windows, you can see on the floating window of the "desktop" the front window keeps changing.
These are all really just a function of (2), that Facebook doesn't own the desktop OS.
The same problem would exist with the Mac. Apple, since they control Mac, could maybe make it work but it would be a huge change to MacOS. They'd effectively need each app to run on its own screen and each screen to be a different resolution and device pixel ratio. Plus they'd need to map in all of the other features to mac MacOS to be usable in AR