async _transitionShow(showConfig) {
    // `this.transitionShow` is a hook for our users
    await this.transitionShow({ backdropNode: this.backdropNode, contentNode: this.contentNode });

    if (showConfig.backdropNode) {
      showConfig.backdropNode.classList.add(`overlays__backdrop--animation-in`);
    }
  }
@param {{backdropNode:HTMLElement; contentNode:HTMLElement}} showConfig