notes.js: use the original content when resetting the form

Otherwise, when an error occurred, the content was escaped and
re-escaped on every error.

Fixes #37724

app/assets/javascripts/notes.js

@@ -1417,7 +1417,7 @@ export default class Notes {
     const isMainForm = $form.hasClass('js-main-target-form');
     const isDiscussionForm = $form.hasClass('js-discussion-note-form');
     const isDiscussionResolve = $submitBtn.hasClass('js-comment-resolve-button');
-    const { formData, formContent, formAction } = this.getFormData($form);
+    const { formData, formContent, formAction, formContentOriginal } = this.getFormData($form);
     let noteUniqueId;
     let systemNoteUniqueId;
     let hasQuickActions = false;
@@ -1576,7 +1576,7 @@ export default class Notes {
           $form = $notesContainer.parent().find('form');
         }
-        $form.find('.js-note-text').val(formContent);
+        $form.find('.js-note-text').val(formContentOriginal);
         this.reenableTargetFormSubmitButton(e);
         this.addNoteError($form);
       });

spec/javascripts/notes_spec.js

@@ -103,6 +103,16 @@ import '~/notes';
         $('.js-comment-button').click();
         expect(this.autoSizeSpy).toHaveBeenTriggered();
       });
+
+      it('should not place escaped text in the comment box in case of error', function() {
+        const deferred = $.Deferred();
+        spyOn($, 'ajax').and.returnValue(deferred.promise());
+        $(textarea).text('A comment with `markup`.');
+
+        deferred.reject();
+        $('.js-comment-button').click();
+        expect($(textarea).val()).toEqual('A comment with `markup`.');
+      });
     });
     describe('updateNote', () => {