ensure mpa navigations to the same URL work after restoring from bfca…

…che (#63155)

### What
When triggering an MPA navigation (also commonly referred to as a "hard
navigation"), and then restoring the previous page via the browser's
bfcache, subsequent requests to the same link wouldn't navigate until
reloading the page or performing a different navigation.

### Why
MPA navigations in app router are handled in a fairly unconventional
way: the router state is updated with an indication that an external URL
was clicked, and once the router sees the pending navigation, it kicks
off a `location.replace` or `location.push` with the specified URL **in
render**. The router then suspends indefinitely to prevent committing
the render. However, the router will only make the `replace`/`push`
request if there's not already a pending navigation to that same URL.

The pending check is needed to avoid continuously calling
`push`/`replace` when unrelated router state changes occur (for example,
if I hover over a link and trigger a prefetch action and the router
re-renders, it shouldn't make another `location.push` call to the same
URL that's pending)

However, the source of the bug is that the variable that holds this
pending state is also restored by the browser's cache, since it takes a
snapshot prior to exiting the page. This means that when clicking the
browser back button, `pendingMpaPath` would still be set to the URL we
just came from. When clicking the link again, it would see that the
requested URL is the same as the pending URL, and not perform any
history actions.

### How
This clears the pending value when the router is restored from bfcache.

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x-ref](https://vercel.slack.com/archives/C0676QZBWKS/p1710169967246929)

Closes NEXT-2781
Closes NEXT-2776

packages/next/src/client/components/app-router.tsx

@@ -449,6 +449,11 @@ function Router({
         return
       }
 
+      // Clear the pendingMpaPath value so that a subsequent MPA navigation to the same URL can be triggered.
+      // This is necessary because if the browser restored from bfcache, the pendingMpaPath would still be set to the value
+      // of the last MPA navigation.
+      globalMutable.pendingMpaPath = undefined
+
       dispatch({
         type: ACTION_RESTORE,
         url: new URL(window.location.href),

test/production/bfcache-routing/index.test.ts

@@ -69,5 +69,15 @@ describe('bfcache-routing', () => {
     // we should be back on the test page with no errors
     html = await browser.evalAsync('document.documentElement.innerHTML')
     expect(html).toContain('BFCache Test')
+
+    // After restoring from bfcache, a subsequent mpa navigation to the same URL should work
+    // We trigger the click via `evalAsync` because when restoring from bfcache, our internal
+    // 'waitForElementByCss' method doesn't think the element is attached to the DOM.
+    await browser.evalAsync(
+      `document.querySelector('a[href="https://example.vercel.sh"]').click()`
+    )
+    await browser.waitForCondition(
+      'window.location.origin === "https://example.vercel.sh"'
+    )
   })
 })