Properly adjust for drag actors which were allocated using a custom
vfunc_allocate() and might not have gotten allocated their preferred
size. When DND reparents the actor to the uiGroup, the drag actor will
get allocated its preferred size, so we also need to take the difference
between the old allocation size and the preferred size into account
before reparenting to the uiGroup.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1310
82da73baff