When entering immersive mode mash creates a new Window to host the reveal. Currently this window is created as a sibling of the window immersive mode is shown for. This is problematic in so far as the rest of ash is likely not going to expect this window to exist in the same container (for example, I suspect it'll get a shelf entry).

Ideally the reveal window should be a child of the window in immersive. But mus disallows this. Specifically once a client has been embedded in a window then owner of the window can no longer add children to that window. We need to relax this constrain (for the windowmanager) in order to get the reveal to work correctly.