async __updateContentSize() {
    if (this._contentNode) {
      if (this.opened) {
        this._contentNode.style.setProperty('display', '');
        await this._showAnimation({ contentNode: this._contentNode });
      } else {
        await this._hideAnimation({ contentNode: this._contentNode });
        this._contentNode.style.setProperty('display', 'none');
      }
    }
  }
Toggle extra content visibility on state change.
@private