Right but xmonad is very idiosyncratic in that behaviour. I don't know of any other window manager that behaves like that, and while it seems useful in Firefox,

Right but xmonad is very idiosyncratic in that behaviour. I don't know of any other window manager that behaves like that, and while it seems useful in Firefox, it doesn't make much sense to ignore fullscreen requests from an application like mpv which has no chrome anyway, or say a video game which expects to be fullscreen and doesn't know how to dynamically scale its UI down when the window isn't the size it expects. PiP seems to be a cleaner solution.