test(e2e): accept beforeunload prompt in shared dialog fallback (#9122)

The USE_REMOTE crash-resume spec reloads Client B mid-sync. While
isSyncInProgress is set, startup.service's beforeunload handler calls
preventDefault(), so Chrome raises a beforeunload prompt (the preceding
button click supplies the user activation it requires). Playwright
auto-accepts such prompts only when no 'dialog' listener is registered —
but installDevErrorDialogHandler registers one on every page and early-
returned on beforeunload, leaving the prompt unanswered. The navigation
then never starts and reload() times out (observed on SuperSync shard
6/6; the earlier load→domcontentloaded de-flake could not help because
navigation is blocked before any lifecycle event).

Accept beforeunload in the shared fallback, restoring Playwright's
default; other dialog types keep flowing to spec-specific handlers.
Corrects the two crash-resume comments that encoded the wrong theory.

e2e/tests/sync/supersync-use-remote-crash-resume.spec.ts

@@ -97,11 +97,8 @@ test.describe('@supersync USE_REMOTE interrupted rebuild recovery', () => {
           rebuildCommittedLog: REBUILD_COMMITTED_LOG,
         },
       );
-      // Wait only for `domcontentloaded`, not the default `load`: an active
+      // `reload()` preserves sessionStorage (unlike close()+newPage()), which
-      // SuperSync WebSocket/sync connection can keep the page "loading" so the
+      // this test needs; `waitForAppReady` below is the real readiness gate.
-      // `load` event never fires and `reload()` times out (flaky). `reload()`
-      // preserves sessionStorage (unlike close()+newPage()), which this test
-      // needs, and `waitForAppReady` below is the real readiness gate.
       await clientB.page.reload({ waitUntil: 'domcontentloaded', timeout: 60000 });
       await waitForAppReady(clientB.page);
 
@@ -136,10 +133,12 @@ test.describe('@supersync USE_REMOTE interrupted rebuild recovery', () => {
 
       await clientB.sync.syncImportUseRemoteBtn.click();
       await crashObserved;
-      // domcontentloaded (not the default `load`) — see the note on the first
+      // This reload races the crashed sync's unwind: while `isSyncInProgress` is
-      // reload above: an active sync connection can block `load` and hang
+      // still set, startup.service's beforeunload handler prevents the unload and
-      // `reload()`. sessionStorage survives the reload; the assertion below
+      // Chrome prompts (the click above supplies the required user activation).
-      // depends on it.
+      // installDevErrorDialogHandler accepts that prompt — without it the
+      // navigation never starts and this hangs. sessionStorage survives the
+      // reload; the assertion below depends on it.
       await clientB.page.reload({ waitUntil: 'domcontentloaded', timeout: 60000 });
       await waitForAppReady(clientB.page);
       expect(

e2e/utils/runtime-errors.ts

@@ -37,6 +37,21 @@ export const attachPageErrorCollector = (
 // explicitly, or it will be vulnerable to the same hang.
 export const installDevErrorDialogHandler = (page: Page, label: string): void => {
   page.on('dialog', async (dialog) => {
+    // Registering ANY 'dialog' listener switches off Playwright's built-in
+    // handling (accept for beforeunload, dismiss for everything else). This
+    // fallback is installed on every page, so it must restore that default for
+    // beforeunload: startup.service raises one whenever a sync is in flight, and
+    // an unanswered prompt blocks the navigation before it starts — reload()
+    // then times out on any waitUntil, however early.
+    if (dialog.type() === 'beforeunload') {
+      try {
+        await dialog.accept();
+      } catch {
+        // Already handled by another listener — ignore.
+      }
+      return;
+    }
+
     const message = dialog.message();
     const isDevErrorAlert = dialog.type() === 'alert' && message.startsWith('devERR:');
     const isDevErrorConfirm =