This might be useful on Wayland where positioning the popup is otherwise
not possible at all. Combined with a session restoration mechanism this
might actually be a feasible workaround. The user just needed to position
the popup once wherever it is wanted to show up and then the compositor
could remember that position.
Especially the resizing would also be useful on other platforms (and is by
the way also already possible when using the Plasmoid showing it as part of
the system tray Plasmoid).