Quoting the spec for `wl_data_device::drop`:
> If the resulting action is "ask", the action will not be considered
> final. The drag-and-drop destination is expected to perform one last
> wl_data_offer.set_actions request, or wl_data_offer.destroy in order
> to cancel the operation.
We did not respect the action choosen by the drop destination when
it called `wl_data_offer::set_actions` after `wl_data_device::drop`
if a user override was still active. This eventually resulted in
a protocol error in `wl_data_offer::finish`, as the current action
could still be `ask`.
Fix this by only allowing a user override to `ask` before `drop` is
called, thus making sure the final `set_actions` preference is
honored.
Closes https://gitlab.gnome.org/GNOME/mutter/-/issues/1952
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2043>
This is still an openly defined struct, as we will need accessed
by "subclasses". Same principle applies than with the
MetaWaylandDataSource refactor, this is not meant to introduce
functional changes, so just go with it.
On the bright side, the interactions are now clearer, so it could
be made saner in the future.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1193
