Despite what I thought in the past, it is allowed, as long as we follow
certain limitations depending on the toplevel's state.
As usual I peppered the code with comments expaining what those
limitations are.
Regarding popups, AFAICT there are no major limitations, although we
should eventually use the new `reposition` method, which autoadjusts the
popup to follow the screen's borders and whatnot. I didn't do that in
this patch as it requires some elbow grease, especially if we want to do
it synchronously.
8a448032c2