We must emit ::dnd-leave to pair the ::dnd-enter that shall be
emitted whenever the plugin grab begins, otherwise we leave
listeners unable to clean up if the plugin begins and ends a
grab while there is an ongoing DnD operation.
https://bugzilla.gnome.org/show_bug.cgi?id=784545
No XDnD events which notify DnD status change comes in Wayland. To emulate XDnD
behavior, MetaDnd checks whether there is a grab or not when the modal window
starts showing. When there is a grab, it processes the raw events from
compositor, and emits DnD signals for plugin.
https://bugzilla.gnome.org/show_bug.cgi?id=765003
Implement MetaDnd for emitting DnD signals to plugins such as gnome-shell. The
xdnd handling code comes from gnome-shell, and it is hidden behind MetaDnd now.
https://bugzilla.gnome.org/show_bug.cgi?id=765003
