Fix race condition of `menu-expanded` and top nav

app/assets/javascripts/main.js

@@ -203,6 +203,9 @@ document.addEventListener('DOMContentLoaded', () => {
   });
 
   $('.navbar-toggler').on('click', () => {
+    // The order is important. The `menu-expanded` is used as a source of truth for now.
+    // This can be simplified when the :combined_menu feature flag is removed.
+    // https://gitlab.com/gitlab-org/gitlab/-/issues/333180
     $('.header-content').toggleClass('menu-expanded');
     navEventHub.$emit(EVENT_RESPONSIVE_TOGGLE);
   });

app/assets/javascripts/nav/components/responsive_app.vue

@@ -3,7 +3,7 @@ import { FREQUENT_ITEMS_PROJECTS, FREQUENT_ITEMS_GROUPS } from '~/frequent_items
 import { BV_DROPDOWN_SHOW, BV_DROPDOWN_HIDE } from '~/lib/utils/constants';
 import KeepAliveSlots from '~/vue_shared/components/keep_alive_slots.vue';
 import eventHub, { EVENT_RESPONSIVE_TOGGLE } from '../event_hub';
-import { resetMenuItemsActive } from '../utils/reset_menu_items_active';
+import { resetMenuItemsActive, hasMenuExpanded } from '../utils';
 import ResponsiveHeader from './responsive_header.vue';
 import ResponsiveHome from './responsive_home.vue';
 import TopNavContainerView from './top_nav_container_view.vue';
@@ -33,9 +33,11 @@ export default {
     },
   },
   created() {
-    eventHub.$on(EVENT_RESPONSIVE_TOGGLE, this.onToggle);
+    eventHub.$on(EVENT_RESPONSIVE_TOGGLE, this.updateResponsiveOpen);
     this.$root.$on(BV_DROPDOWN_SHOW, this.showMobileOverlay);
     this.$root.$on(BV_DROPDOWN_HIDE, this.hideMobileOverlay);
+
+    this.updateResponsiveOpen();
   },
   beforeDestroy() {
     eventHub.$off(EVENT_RESPONSIVE_TOGGLE, this.onToggle);
@@ -43,8 +45,12 @@ export default {
     this.$root.$off(BV_DROPDOWN_HIDE, this.hideMobileOverlay);
   },
   methods: {
-    onToggle() {
-      document.body.classList.toggle('top-nav-responsive-open');
+    updateResponsiveOpen() {
+      if (hasMenuExpanded()) {
+        document.body.classList.add('top-nav-responsive-open');
+      } else {
+        document.body.classList.remove('top-nav-responsive-open');
+      }
     },
     onMenuItemClick({ view }) {
       if (view) {

app/assets/javascripts/nav/utils/has_menu_expanded.js

+export const hasMenuExpanded = () => {
+  const header = document.querySelector('.header-content');
+
+  return Boolean(header?.classList.contains('menu-expanded'));
+};

app/assets/javascripts/nav/utils/index.js

+export * from './has_menu_expanded';
+export * from './reset_menu_items_active';

spec/frontend/nav/components/responsive_app_spec.js

 import { shallowMount } from '@vue/test-utils';
-import { range } from 'lodash';
 import ResponsiveApp from '~/nav/components/responsive_app.vue';
 import ResponsiveHeader from '~/nav/components/responsive_header.vue';
 import ResponsiveHome from '~/nav/components/responsive_home.vue';
@@ -32,6 +31,7 @@ describe('~/nav/components/responsive_app.vue', () => {
   const hasMobileOverlayVisible = () => findMobileOverlay().classes('mobile-nav-open');
 
   beforeEach(() => {
+    document.body.innerHTML = '';
     // Add test class to reset state + assert that we're adding classes correctly
     document.body.className = 'test-class';
   });
@@ -53,14 +53,17 @@ describe('~/nav/components/responsive_app.vue', () => {
     });
 
     it.each`
-      times | expectation
-      ${0}  | ${false}
-      ${1}  | ${true}
-      ${2}  | ${false}
+      bodyHtml                                                          | expectation
+      ${''}                                                             | ${false}
+      ${'<div class="header-content"></div>'}                           | ${false}
+      ${'<div class="menu-expanded"></div>'}                            | ${false}
+      ${'<div></div><div class="header-content menu-expanded"></div>}'} | ${true}
     `(
-      'with responsive toggle event triggered $times, body responsive open = $expectation',
-      ({ times, expectation }) => {
-        range(times).forEach(triggerResponsiveToggle);
+      'with responsive toggle event and html set to $bodyHtml, responsive open = $expectation',
+      ({ bodyHtml, expectation }) => {
+        document.body.innerHTML = bodyHtml;
+
+        triggerResponsiveToggle();
 
         expect(hasBodyResponsiveOpen()).toBe(expectation);
       },
@@ -88,6 +91,17 @@ describe('~/nav/components/responsive_app.vue', () => {
     );
   });
 
+  describe('with menu expanded in body', () => {
+    beforeEach(() => {
+      document.body.innerHTML = '<div></div><div class="header-content menu-expanded"></div>';
+      createComponent();
+    });
+
+    it('sets the body responsive open', () => {
+      expect(hasBodyResponsiveOpen()).toBe(true);
+    });
+  });
+
   const projectsContainerProps = {
     containerClass: 'gl-px-3',
     frequentItemsDropdownType: ResponsiveApp.FREQUENT_ITEMS_PROJECTS.namespace,