Attach user popovers to DOM nodes added after the initial DOM query

app/assets/javascripts/user_popovers.js

@@ -59,11 +59,33 @@ const populateUserInfo = (user) => {
 };
 
 const initializedPopovers = new Map();
+let domObservedForChanges = false;
 
-export default (elements = document.querySelectorAll('.js-user-link')) => {
+const addPopoversToModifiedTree = new MutationObserver(() => {
+  const userLinks = document?.querySelectorAll('.js-user-link, .gfm-project_member');
+
+  if (userLinks) {
+    addPopovers(userLinks); /* eslint-disable-line no-use-before-define */
+  }
+});
+
+function observeBody() {
+  if (!domObservedForChanges) {
+    addPopoversToModifiedTree.observe(document.body, {
+      subtree: true,
+      childList: true,
+    });
+
+    domObservedForChanges = true;
+  }
+}
+
+export default function addPopovers(elements = document.querySelectorAll('.js-user-link')) {
   const userLinks = Array.from(elements);
   const UserPopoverComponent = Vue.extend(UserPopover);
 
+  observeBody();
+
   return userLinks
     .filter(({ dataset }) => dataset.user || dataset.userId)
     .map((el) => {
@@ -105,4 +127,4 @@ export default (elements = document.querySelectorAll('.js-user-link')) => {
 
       return renderedPopover;
     });
-};
+}

changelogs/unreleased/tor-defect-missing-popovers-from-async-DOM.yml

+---
+title: React to new DOM nodes being added to the page to bind the user information
+  popover to them
+merge_request: 54411
+author:
+type: fixed

spec/frontend/user_popovers_spec.js

@@ -6,6 +6,19 @@ describe('User Popovers', () => {
   preloadFixtures(fixtureTemplate);
 
   const selector = '.js-user-link, .gfm-project_member';
+  const findFixtureLinks = () => {
+    return Array.from(document.querySelectorAll(selector)).filter(
+      ({ dataset }) => dataset.user || dataset.userId,
+    );
+  };
+  const createUserLink = () => {
+    const link = document.createElement('a');
+
+    link.classList.add('js-user-link');
+    link.setAttribute('data-user', '1');
+
+    return link;
+  };
 
   const dummyUser = { name: 'root' };
   const dummyUserStatus = { message: 'active' };
@@ -37,13 +50,20 @@ describe('User Popovers', () => {
   });
 
   it('initializes a popover for each user link with a user id', () => {
-    const linksWithUsers = Array.from(document.querySelectorAll(selector)).filter(
-      ({ dataset }) => dataset.user || dataset.userId,
-    );
+    const linksWithUsers = findFixtureLinks();
 
     expect(linksWithUsers.length).toBe(popovers.length);
   });
 
+  it('adds popovers to user links added to the DOM tree after the initial call', async () => {
+    document.body.appendChild(createUserLink());
+    document.body.appendChild(createUserLink());
+
+    const linksWithUsers = findFixtureLinks();
+
+    expect(linksWithUsers.length).toBe(popovers.length + 2);
+  });
+
   it('does not initialize the user popovers twice for the same element', () => {
     const newPopovers = initUserPopovers(document.querySelectorAll(selector));
     const samePopovers = popovers.every((popover, index) => newPopovers[index] === popover);