Gtk4 DND button not available in drop handlers
In DropTarget.motion and DropTarget.drop handlers, trying to determine which button is being pressed by examining the ModifierType returned by DropTarget.get_current_event_state () or from the default seat's keyboard or pointer failed although the keyboard modifiers were available. This information only seemed available in the DragSource.prepare handler (the current event state was always zero in the DragSource.begin handler). As a workaround the button was stored and then added as data to the Drag object.
The modifier state of the motion and drop events should reflect the button being used for dragging and it would be convenient if this were also a property of the Drop and/or Drag objects.