The ongoing drop operation over the controller's widget or its descendant.
If no drop operation is going on, this property returns null.
The event controller should not modify the
drop, but it might want to query its properties.
When handling crossing events, this property is updated before [signal@Gtk.DropControllerMotion:
but after [signal@Gtk.DropControllerMotion:
GtkDropControllerMotion:leave] is emitted.