code: function _transitionComplete() {
      if ( element.classList.contains( CLASSES.EXPANDED ) ) {
        this.dispatchEvent( 'expandEnd', { target: this } );
  
        if ( element.scrollHeight > previousHeight ) {
          element.style.maxHeight = element.scrollHeight + 'px';
        }
      } else if ( element.classList.contains( CLASSES.COLLAPSED ) ) {
        this.dispatchEvent( 'collapseEnd', { target: this } );
      }
    }
docs: Handle the end of a transition.