App Router: fix relative query/hash handling in next/link and route…

…r push/replace (vercel#49521)

## Problem
Relative hash/query handling in `next/link` (e.g. `<Link
href="#hello">`) is broken in App Router, especially if you're on a
nested route.

This wasn't a problem in `/pages` because the href always get fully
resolved in `<NextLink>`; i.e. if you have `<Link href="#hash" />` on
`/hello`, it'll resolve the href to `/hello#hash` and use that
everywhere.

However, in App Router, `<Link>` no longer uses the current location to
resolve the href:
https://github.com/vercel/next.js/blob/5451564f364399003b05939fa6dd7d32c1dabec7/packages/next/src/client/link.tsx#L450-L457

Therefore navigating with `new URL(href, location.origin)` will skip the
current path and always apply the relative hash/query to the _root_
route:
https://github.com/vercel/next.js/blob/5451564f364399003b05939fa6dd7d32c1dabec7/packages/next/src/client/components/app-router.tsx#L208-L215

## Solution
Not 100% sure if this is the best solution, but since `app-router` is
already reading `window.location`, I'm using `location.href` as the base
URL to resolve the href.

I grep'd for `location.origin` and checked other callsites; seems like
only `app-router` deals with user specified hrefs.

Fixes vercel#42157 &
vercel#44139, and potentially
vercel#48554 and
vercel#22838

## Test Plan
```
pnpm testheadless test/e2e/app-dir/navigation
```

---------

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

@@ -211,7 +211,7 @@ function Router({
       navigateType: 'push' | 'replace',
       forceOptimisticNavigation: boolean
     ) => {
-      const url = new URL(addBasePath(href), location.origin)
+      const url = new URL(addBasePath(href), location.href)
 
       return dispatch({
         type: ACTION_NAVIGATE,
@@ -261,7 +261,7 @@ function Router({
         if (isBot(window.navigator.userAgent)) {
           return
         }
-        const url = new URL(addBasePath(href), location.origin)
+        const url = new URL(addBasePath(href), location.href)
         // External urls can't be prefetched in the same way.
         if (isExternalURL(url)) {
           return

test/e2e/app-dir/navigation/app/hash/page.js

@@ -20,6 +20,9 @@ export default function HashPage() {
       <Link href="/hash#hash-300" id="link-to-300">
         To 300
       </Link>
+      <Link href="#hash-500" id="link-to-500">
+        To 500 (hash only)
+      </Link>
       <Link href="/hash#top" id="link-to-top">
         To Top
       </Link>

test/e2e/app-dir/navigation/app/nested-relative-query-and-hash/client-component.js

@@ -0,0 +1,20 @@
+'use client'
+
+import { useRouter } from 'next/navigation'
+
+export function RouterPushButton() {
+  const router = useRouter()
+
+  return (
+    <h3 id="h3">
+      <button
+        id="button-to-h3-hash-only"
+        onClick={() => {
+          router.push('#h3')
+        }}
+      >
+        To #h3, hash only
+      </button>
+    </h3>
+  )
+}

test/e2e/app-dir/navigation/app/nested-relative-query-and-hash/page.js

@@ -0,0 +1,28 @@
+import Link from 'next/link'
+import { RouterPushButton } from './client-component'
+
+export default function Page() {
+  return (
+    <>
+      <h1 id="h1">
+        <Link href="#h1" id="link-to-h1-hash-only">
+          To #h1, hash only
+        </Link>
+      </h1>
+
+      <p>
+        <Link href="?foo=1&bar=2" id="link-to-dummy-query">
+          Query only
+        </Link>
+      </p>
+
+      <h2 id="h2">
+        <Link href="?here=ok#h2" id="link-to-h2-with-hash-and-query">
+          To #h2, with both relative hash and query
+        </Link>
+      </h2>
+
+      <RouterPushButton />
+    </>
+  )
+}

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

@@ -52,6 +52,7 @@ createNextDescribe(
         await checkLink(50, 730)
         await checkLink(160, 2270)
         await checkLink(300, 4230)
+        await checkLink(500, 7030) // this one is hash only (`href="#hash-500"`)
         await checkLink('top', 0)
         await checkLink('non-existent', 0)
       })
@@ -106,6 +107,60 @@ createNextDescribe(
       })
     })
 
+    describe('relative hashes and queries', () => {
+      const pathname = '/nested-relative-query-and-hash'
+
+      it('should work with a hash-only href', async () => {
+        const browser = await next.browser(pathname)
+        await browser.elementByCss('#link-to-h1-hash-only').click()
+
+        await check(() => browser.url(), next.url + pathname + '#h1')
+      })
+
+      it('should work with a hash-only `router.push(...)`', async () => {
+        const browser = await next.browser(pathname)
+        await browser.elementByCss('#button-to-h3-hash-only').click()
+
+        await check(() => browser.url(), next.url + pathname + '#h3')
+      })
+
+      it('should work with a query-only href', async () => {
+        const browser = await next.browser(pathname)
+        await browser.elementByCss('#link-to-dummy-query').click()
+
+        await check(() => browser.url(), next.url + pathname + '?foo=1&bar=2')
+      })
+
+      it('should work with both relative hashes and queries', async () => {
+        const browser = await next.browser(pathname)
+        await browser.elementByCss('#link-to-h2-with-hash-and-query').click()
+
+        await check(() => browser.url(), next.url + pathname + '?here=ok#h2')
+
+        // Only update hash
+        await browser.elementByCss('#link-to-h1-hash-only').click()
+        await check(() => browser.url(), next.url + pathname + '?here=ok#h1')
+
+        // Replace all with new query
+        await browser.elementByCss('#link-to-dummy-query').click()
+        await check(() => browser.url(), next.url + pathname + '?foo=1&bar=2')
+
+        // Add hash to existing query
+        await browser.elementByCss('#link-to-h1-hash-only').click()
+        await check(
+          () => browser.url(),
+          next.url + pathname + '?foo=1&bar=2#h1'
+        )
+
+        // Update hash again via `router.push(...)`
+        await browser.elementByCss('#button-to-h3-hash-only').click()
+        await check(
+          () => browser.url(),
+          next.url + pathname + '?foo=1&bar=2#h3'
+        )
+      })
+    })
+
     describe('not-found', () => {
       it('should trigger not-found in a server component', async () => {
         const browser = await next.browser('/not-found/servercomponent')