code: function handleWheelEvent( tracker, event, originalEvent ) {
          var nDelta = 0,
              propagate;
  
          // The nDelta variable is gated to provide smooth z-index scrolling
          //   since the mouse wheel allows for substantial deltas meant for rapid
          //   y-index scrolling.
          // event.deltaMode: 0=pixel, 1=line, 2=page
          // TODO: Deltas in pixel mode should be accumulated then a scroll value computed after $.DEFAULT_SETTINGS.pixelsPerWheelLine threshold reached
          nDelta = event.deltaY < 0 ? 1 : -1;
  
          if ( tracker.scrollHandler ) {
              propagate = tracker.scrollHandler(
                  {
                      eventSource:          tracker,
                      pointerType:          'mouse',
                      position:             getMouseRelative( event, tracker.element ),
                      scroll:               nDelta,
                      shift:                event.shiftKey,
                      isTouchEvent:         false,
                      originalEvent:        originalEvent,
                      preventDefaultAction: false,
                      userData:             tracker.userData
                  }
              );
              if ( propagate === false ) {
                  $.cancelEvent( originalEvent );
              }
          }
      }
docs: Handles 'wheel' events.
  The event may be simulated by the legacy mouse wheel event handler (onMouseWheel()).
  
  @private
  @inner