opigno_calendar-8.x-1.7/js/opigno_calendar_form.js

(function ($, Drupal) {
  // Fixes multiselect issue 2123241.
  if (Drupal.behaviors.multiSelect
      && !Drupal.behaviors.multiSelect.detach
  ) {
    Drupal.behaviors.multiSelect.detach = function (context, settings, trigger) {
      if (trigger === 'serialize') {
        $('select.multiselect-selected').selectAll();
      }
    };
  }
}(jQuery, Drupal));