This method returns zero-based index of the track item for which the user modified the highlighting state. The item is either a track vertex or segment, depending on whether the client has synchronously called setHighlightVertexMode or setHighlightSegmentMode, respectively. The returned value is valid only after isSynchronousOpComplete returns TRUE.
To determine which track contains the modified item, call getKey. To determine whether highlighting was turned on or off, call getItemState.