fix(next/client): keep hash when navigating from app to pages router (#56223)

### What?

Fixes the pages router not receiving a hash when being linked from the
app router.

### Why?

The hash being removed breaks sites that rely on it for client side
features.

### How?

The hash gets omitted from the URL when used as a key for the
preflightCache. Once it's clear that it's an external URL and that it's
not empty we can use the initial href to send the hash as well.

Not completely sure if there's an edge case that might break, I added an
extra check for when the hash is only used to scroll the page.

This might need an additional test case just for
`navigate-reducer.test.tsx`.

Fixes #56212

---------

Co-authored-by: Zack Tanner <zacktanner@gmail.com>
Co-authored-by: JJ Kasper <jj@jjsweb.site>

Author: 3 people

packages/next/src/client/components/router-reducer/fetch-server-response.ts

@@ -122,6 +122,11 @@ export async function fetchServerResponse(
     // If fetch returns something different than flight response handle it like a mpa navigation
     // If the fetch was not 200, we also handle it like a mpa navigation
     if (!isFlightResponse || !res.ok) {
+      // in case the original URL came with a hash, preserve it before redirecting to the new URL
+      if (url.hash) {
+        responseUrl.hash = url.hash
+      }
+
       return doMpaNavigation(responseUrl.toString())
     }
 

test/e2e/app-dir/navigation/app/hash-link-to-pages-router/global.css

@@ -0,0 +1,7 @@
+* {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+  font-size: 14px;
+  line-height: 1;
+}

test/e2e/app-dir/navigation/app/hash-link-to-pages-router/page.js

@@ -0,0 +1,13 @@
+import Link from 'next/link'
+import './global.css'
+
+export default function HashPage() {
+  return (
+    <div style={{ fontFamily: 'sans-serif', fontSize: '16px' }}>
+      <p>Hash To Pages Router Page</p>
+      <Link href="/some#non-existent" id="link-to-pages-router">
+        To pages router
+      </Link>
+    </div>
+  )
+}

test/e2e/app-dir/navigation/navigation.test.ts

@@ -498,6 +498,15 @@ createNextDescribe(
         expect(await browser.url()).toBe(next.url + '/some')
       })
 
+      it('should not omit the hash while navigating from app to pages', async () => {
+        const browser = await next.browser('/hash-link-to-pages-router')
+        await browser
+          .elementByCss('#link-to-pages-router')
+          .click()
+          .waitForElementByCss('#link-to-app')
+        await check(() => browser.url(), next.url + '/some#non-existent')
+      })
+
       if (!isNextDev) {
         // this test is pretty hard to test in playwright, so most of the heavy lifting is in the page component itself
         // it triggers a hover on a link to initiate a prefetch request every second, and so we check that