Issue #2291439 by Wim Leers, tstoeckler: Fixed CKEditor's "readonly" needs to be per-instance but is applied per-format, across instances.

core/modules/ckeditor/js/ckeditor.js

@@ -14,10 +14,8 @@
       // CKEditor initializes itself in a read-only state if the 'disabled'
       // attribute is set. It does not respect the 'readonly' attribute,
       // however, so we set the 'readOnly' configuration property manually in
-      // that case.
-      if (element.hasAttribute('readonly')) {
-        format.editorSettings.readOnly = true;
-      }
+      // that case, for the CKEditor instance that's about to be created.
+      format.editorSettings.readOnly = element.hasAttribute('readonly');
 
       return !!CKEDITOR.replace(element, format.editorSettings);
     },