field_group-8.x-3.1/js/field_group.tab_validation.js

(function ($, once) {
  /**
   * Invalid event handler for input elements in Tab field group.
   */
  const onTabInvalid = function (e) {
    $(e.target)
      .parents('details ')
      .children(
        'summary[aria-expanded=false]:not(.horizontal-tabs-pane > summary, .vertical-tabs__pane > summary)',
      )
      .parent()
      .attr('open', 'open');
  };

  /**
   * Make sure tab field groups which contain invalid data are expanded when
   * they first load, and also when someone clicks the submit button.
   */
  Drupal.behaviors.fieldGroupTabValidation = {
    attach(context) {
      $(
        once(
          'field-group-tab-validation',
          $('.field-group-tab :input', context),
        ),
      ).on('invalid.field_group', onTabInvalid);
    },
  };
})(jQuery, once);