Source: themes/seven/js/nav-tabs.es6.js

/**
 * @file
 * Responsive navigation tabs.
 *
 * This also supports collapsible navigable is the 'is-collapsible' class is
 * added to the main element, and a target element is included.
 */
(function ($, Drupal) {
  function init(i, tab) {
    const $tab = $(tab);
    const $target = $tab.find('[data-drupal-nav-tabs-target]');
    const isCollapsible = $tab.hasClass('is-collapsible');

    function openMenu(e) {
      $target.toggleClass('is-open');
    }

    function handleResize(e) {
      $tab.addClass('is-horizontal');
      const $tabs = $tab.find('.tabs');
      const isHorizontal =
        $tabs.outerHeight() <= $tabs.find('.tabs__tab').outerHeight();
      $tab.toggleClass('is-horizontal', isHorizontal);
      if (isCollapsible) {
        $tab.toggleClass('is-collapse-enabled', !isHorizontal);
      }
      if (isHorizontal) {
        $target.removeClass('is-open');
      }
    }

    $tab.addClass('position-container is-horizontal-enabled');

    $tab.on('click.tabs', '[data-drupal-nav-tabs-trigger]', openMenu);
    $(window)
      .on('resize.tabs', Drupal.debounce(handleResize, 150))
      .trigger('resize.tabs');
  }

  /**
   * Initialize the tabs JS.
   */
  Drupal.behaviors.navTabs = {
    attach(context, settings) {
      const $tabs = $(context).find('[data-drupal-nav-tabs]');
      if ($tabs.length) {
        const notSmartPhone = window.matchMedia('(min-width: 300px)');
        if (notSmartPhone.matches) {
          $tabs.once('nav-tabs').each(init);
        }
      }
    },
  };
})(jQuery, Drupal);