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

core/modules/ckeditor5/js/ckeditor5.es6.js

@@ -4,6 +4,13 @@
  */
 /* 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.
    *

core/modules/ckeditor5/js/ckeditor5.js

@@ -6,6 +6,10 @@
 **/
 
 ((Drupal, debounce, CKEditor5, $, once) => {
+  if (!CKEditor5) {
+    return;
+  }
+
   Drupal.CKEditor5Instances = new Map();
   const callbacks = new Map();
   const required = new Set();

core/modules/ckeditor5/js/ie11.user.warnings.es6.js

@@ -14,6 +14,11 @@
   // 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.

core/modules/ckeditor5/js/ie11.user.warnings.js

@@ -9,6 +9,7 @@
   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) {