code: function(ev) {
              // ignore right clicks
              if (ev.which !== Garnish.PRIMARY_CLICK) {
                  return;
              }
  
              // Enforce the filter
              if (this.settings.filter && !$(ev.target).is(this.settings.filter)) {
                  return;
              }
  
              this.mousedownTarget = ev.currentTarget;
  
              var $item = $($.data(ev.currentTarget, 'select-item'));
  
              if (this.first !== null && ev.shiftKey) {
                  // Shift key is consistent for both selection modes
                  this.selectRange($item, true);
              }
              else if (this._actAsCheckbox(ev)) {
                  this.toggleItem($item, true);
              }
          }
docs: Events --------------------------------------------------------------------- 
  On Mouse Down