Function begin_in_app_drag 

pub fn begin_in_app_drag(
    starts: MessageReader<'_, '_, DragStartEvent>,
    sources: Query<'_, '_, &DragSource>,
    active: ResMut<'_, ActiveDrag>,
    out: MessageWriter<'_, DragStarted>,
)

Open an ActiveDrag session and announce DragStarted when a drag gesture crosses the threshold on a DragSource entity. Mirrors QDrag::exec beginning a drag with the source’s QMimeData / HTML5 dragstart populating dataTransfer.