Fix test.

The event listener doesn't block the promise, so the asserts inside the
listener don't run without this change (it's trivial to check by
expanding the asserts in the test summary).

Differential Revision: https://phabricator.services.mozilla.com/D217200

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1908907
gecko-commit: ba4dc8ba4f47a10c94d5c738afd0099a53060774
gecko-reviewers: jjaschke

scroll-to-text-fragment/sequential-focus.html

@@ -24,21 +24,21 @@
             const link = document.getElementById('link');
             const nextButton = document.getElementById('next-button');
 
-            window.addEventListener('hashchange', async () => {
-                // Assert that the link element does not have focus initially
-                assert_not_equals(document.activeElement, link, 'Link element should not have focus initially');
+            const hashChange = new Promise(r => window.addEventListener('hashchange', r, { once: true }));
 
-                // Simulate pressing TAB to shift focus to the next element
-                await test_driver.send_keys(document.body, '\uE004'); // '\uE004' is the WebDriver key code for TAB
+            // Simulate a same-document load to the text fragment
+            location.hash = '#some-hash-to-trigger-hashchange:~:text=foo';
 
-                // Assert that the next focusable element (button) has focus
-                assert_equals(document.activeElement, nextButton, 'Next focusable element should have focus after pressing TAB');
+            await hashChange;
 
-                t.done();
-            });
+            // Assert that the link element does not have focus initially
+            assert_not_equals(document.activeElement, link, 'Link element should not have focus initially');
 
-            // Simulate a same-document load to the text fragment
-            location.hash = '#some-hash-to-trigger-hashchange:~:text=foo';
+            // Simulate pressing TAB to shift focus to the next element
+            await test_driver.send_keys(document.body, '\uE004'); // '\uE004' is the WebDriver key code for TAB
+
+            // Assert that the next focusable element (button) has focus
+            assert_equals(document.activeElement, nextButton, 'Next focusable element should have focus after pressing TAB');
         }, 'Text Fragment focus behavior');
     </script>
 </body>