code: function DropdownEventHandler($list, selectionCallback, closeCallback) {
  
          this.$list = $list;
          this.$items = $list.find("li");
          this.selectionCallback = selectionCallback;
          this.closeCallback = closeCallback;
          this.scrolling = false;
  
          /**
           * @private
           * The selected position in the list; otherwise -1.
           * @type {number}
           */
          this._selectedIndex = -1;
      }
docs: Object to handle events for a dropdown list.
  
  DropdownEventHandler handles these events:
  
  Mouse:
  - click       - execute selection callback and dismiss list
  - mouseover   - highlight item
  - mouseleave  - remove mouse highlighting
  
  Keyboard:
  - Enter       - execute selection callback and dismiss list
  - Esc         - dismiss list
  - Up/Down     - change selection
  - PageUp/Down - change selection
  
  Items whose <a> has the .disabled class do not respond to selection.
  
  @constructor
  @param {jQueryObject} $list  associated list object
  @param {Function} selectionCallback  function called when list item is selected.