This commit completes the implementation of `xdg_wm_base` version 3,
which introduces support for synchronized implicit and explicit popup
repositioning.
Explicit repositioning works by the client providing a new
`xdg_positioner` object via a new request `xdg_popup.reposition`. If the
repositioning is done in combination with the parent itself being
reconfigured, the to be committed state of the parent is provided by the
client via the `xdg_positioner` object, using
`xdg_positioner.set__parent_configure`.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/705
f97804f4f4