Question Where is the control while mouse button is held down but eventData.delta is Vector2.zero?

Im trying to implement IDragHandler, IBeginDragHandler, IEndDragHandler interfaces and ran into an issuse. im relying on the delta vector as my input in OnDrag(PointerEventData eventData) method. but this method does not run when i started to drag but am now holding my mouse cursor still. the control seems as if its in limbo in this situation because its not running OnDrag() neither OnEndDrag(). so i'd like to know where should i check if delta has become zero so i can handle it.
Thanks!