code: function expand() {
      this.dispatchEvent( 'expandBegin', { target: this } );
  
      if ( !previousHeight || element.scrollHeight > previousHeight ) {
        previousHeight = element.scrollHeight;
      }
  
      element.style.maxHeight = previousHeight + 'px';
      _baseTransition.applyClass( CLASSES.EXPANDED );
  
      return this;
    }
docs: Expands the expandable content
  @returns {ExpandableTransition} An instance.