Issue #3266310 by bnjmnm, Wim Leers, longwave: IE11 user warning has ungraceful failures

 */

/* global CKEditor5 */

((Drupal, debounce, CKEditor5, $, once) => {

  // CKEditor 5 is incompatible with IE11. When IE11 is detected, the CKEditor5

  // variable is null. In those instances, exit early since CKEditor 5 is not

  // loaded.

  if (!CKEditor5) {

    return;

  }

  /**

   * The CKEDITOR instances.

   *

**/

((Drupal, debounce, CKEditor5, $, once) => {

  if (!CKEditor5) {

    return;

  }

  Drupal.CKEditor5Instances = new Map();

  const callbacks = new Map();

  const required = new Set();

  // created. Features such as Quick Edit that require the presence of a

  // Drupal.editors.ckeditor5, even for fields that do not use CKEditor 5.

  if (isIE11) {

    // Explicitly set the global CKEditor5 object to null. This ensures code

    // expecting the existence of the object does not fail, but is easily

    // distinguishable from a valid CKEditor5 object.

    window.CKEditor5 = null;

    // This will reference a MutationObserver used by several functions in

    // Drupal.editors.ckeditor5. It is declared here and not the editor object

    // in order to work with IE11 object scope.

  const isIE11 = Modernizr.mq('(-ms-high-contrast: active), (-ms-high-contrast: none)');

  if (isIE11) {

    window.CKEditor5 = null;

    let quickEditLabelObserver = null;

    Drupal.editors.ckeditor5 = {

      attach: function attach(element) {