Issue #3391779: Re-attach maxlength counter behavior after text format change.

Closes #3391779

js/maxlength.js

-(function ($, Drupal) {
+(function ($, Drupal, once) {
 
   // Make variables local.
   var ml = ml || {};
@@ -29,6 +29,30 @@
             ml.ckeditor5(Drupal.CKEditor5Instances.get(eid), options);
           });
         }
+
+        // Set up a mutation observer to re-attach this behavior when the text
+        // format has been changed. Observer will act on changes to the
+        // 'data-ckeditor5-id' attribute on the field.
+        // @see Drupal.editors.ckeditor5.attach() and setElementId() in Drupal core ckeditor5.js
+        if (Drupal.hasOwnProperty('CKEditor5Instances')) {
+          const observerOptions = {
+            attributes: true,
+            attributeFilter: ['data-ckeditor5-id']
+          };
+          const observer = new MutationObserver((mutations) => {
+            mutations.forEach((mutation) => {
+              const eid = mutation.target.dataset.ckeditor5Id;
+              if (!Drupal.CKEditor5Instances.has(eid)) {
+                observer.disconnect();
+                Drupal.behaviors.maxlength.detach(context);
+                Drupal.behaviors.maxlength.attach(context);
+                // Remove `maxlengthbinding` set by ml.ckeditor5().
+                once.remove('maxlengthbinding', mutation.target);
+              }
+            });
+          });
+          observer.observe(this, observerOptions);
+        }
       });
     },
     detach: function(context, settings) {
@@ -371,4 +395,4 @@
     });
   }
 
-})(jQuery, Drupal);
+})(jQuery, Drupal, once);