test(html): cover pages navigated away from before they arrive

Navigating elsewhere before a page arrives aborts the navigation to it,
and the handler drops the page when it does arrive, so only the last
page navigated to shows, however late the earlier ones answer. Without
the `event.signal.aborted` check, this test fails: the late page is
swapped in over the last one.

Assisted-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: ovflowd

e2e/client-side-navigation.spec.js

@@ -149,6 +149,38 @@ test.describe('Client-side navigation', () => {
     expect(await page.evaluate(() => window.__document)).toBeUndefined();
   });
 
+  test('shows only the last of the pages navigated to at once', async ({
+    page,
+  }) => {
+    // The page navigated to first answers last
+    await page.route(/\/all(\.html)?$/, async route => {
+      await new Promise(resolve => setTimeout(resolve, 500));
+      await route.fallback();
+    });
+
+    const answered = page.waitForEvent('requestfinished', request =>
+      /\/all$/.test(request.url())
+    );
+
+    await page.evaluate(() => {
+      window.__swaps = 0;
+
+      new MutationObserver(records => {
+        window.__swaps += records.length;
+      }).observe(document.documentElement, { childList: true });
+
+      navigation.navigate('all.html');
+    });
+
+    await navigate(page, '404.html');
+    await answered;
+    // Gives the first navigation the chance to show its page, if it would
+    await page.evaluate(() => new Promise(requestAnimationFrame));
+
+    await expect(page).toHaveTitle(/^Page Not Found \|/);
+    expect(await page.evaluate(() => window.__swaps)).toBe(1);
+  });
+
   test('leaves the URL to scripts that rewrite it', async ({ page }) => {
     // As a script dropping tracking parameters does
     const swapped = await page.evaluate(async () => {