Fix going back to page after applying hash link (#50006)

## What?

@steven-tey noticed that on upstash.com that scrolling to hash keeps
happening regardless of the hash being set:

- Open site
- Click `Pricing` which adds `#pricing` and scrolls to the
`id="pricing"`
- Click `About` 
- Click on the logo

You'll notice that navigating back to the homepage ends up scrolling to
the `id="pricing` instead of to the top of the page.

## How?

This happened because we didn't reset the `hashFragment` and
`segmentPaths` for scrolling when scroll was applied, which means it
would keep that value in the state and would be applied as such on
navigation.

This PR ensures that besides setting `apply` to `false` we also reset
the `hashFragment` and `segmentPaths`.

Fixes NEXT-1205

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---------

Co-authored-by: JJ Kasper <[email protected]>

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

@@ -211,6 +211,8 @@ class InnerScrollAndFocusHandler extends React.Component<ScrollAndFocusHandlerPr
 
       // State is mutated to ensure that the focus and scroll is applied only once.
       focusAndScrollRef.apply = false
+      focusAndScrollRef.hashFragment = null
+      focusAndScrollRef.segmentPaths = []
 
       handleSmoothScroll(
         () => {

test/e2e/app-dir/navigation/app/hash-link-back-to-same-page/other/page.js

@@ -0,0 +1,12 @@
+import Link from 'next/link'
+
+export default function Page() {
+  return (
+    <>
+      <p>Other Page</p>
+      <Link href="/hash-link-back-to-same-page" id="link-to-home">
+        Back to test home
+      </Link>
+    </>
+  )
+}

test/e2e/app-dir/navigation/app/hash-link-back-to-same-page/page.js

@@ -0,0 +1,50 @@
+import Link from 'next/link'
+import '../hash/global.css'
+
+export default function HashPage() {
+  // Create list of 5000 items that all have unique id
+  const items = Array.from({ length: 5000 }, (_, i) => ({ id: i }))
+
+  return (
+    <div style={{ fontFamily: 'sans-serif', fontSize: '16px' }}>
+      <p>Hash Page</p>
+      <Link href="/hash-link-back-to-same-page#hash-6" id="link-to-6">
+        To 6
+      </Link>
+      <Link href="/hash-link-back-to-same-page#hash-50" id="link-to-50">
+        To 50
+      </Link>
+      <Link href="/hash-link-back-to-same-page#hash-160" id="link-to-160">
+        To 160
+      </Link>
+      <div>
+        {items.map((item) => {
+          if (item.id === 160) {
+            return (
+              <>
+                <div key={item.id}>
+                  <div id={`hash-${item.id}`}>{item.id}</div>
+                </div>
+                <div key="to-other-page">
+                  <div>
+                    <Link
+                      href="/hash-link-back-to-same-page/other"
+                      id="to-other-page"
+                    >
+                      To other page
+                    </Link>
+                  </div>
+                </div>
+              </>
+            )
+          }
+          return (
+            <div key={item.id}>
+              <div id={`hash-${item.id}`}>{item.id}</div>
+            </div>
+          )
+        })}
+      </div>
+    </div>
+  )
+}

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

@@ -39,7 +39,6 @@ createNextDescribe(
           await check(
             async () => {
               const val = await browser.eval('window.pageYOffset')
-              require('console').error({ val })
               return val.toString()
             },
             expectedScroll.toString(),
@@ -58,6 +57,55 @@ createNextDescribe(
       })
     })
 
+    describe('hash-link-back-to-same-page', () => {
+      it('should scroll to the specified hash', async () => {
+        const browser = await next.browser('/hash-link-back-to-same-page')
+
+        const checkLink = async (
+          val: number | string,
+          expectedScroll: number
+        ) => {
+          await browser.elementByCss(`#link-to-${val.toString()}`).click()
+          await check(
+            async () => {
+              const val = await browser.eval('window.pageYOffset')
+              return val.toString()
+            },
+            expectedScroll.toString(),
+            true,
+            // Try maximum of 15 seconds
+            15
+          )
+        }
+
+        await checkLink(6, 114)
+        await checkLink(50, 730)
+        await checkLink(160, 2270)
+
+        await browser
+          .elementByCss('#to-other-page')
+          // Navigate to other
+          .click()
+          // Wait for other ot load
+          .waitForElementByCss('#link-to-home')
+          // Navigate back to hash-link-back-to-same-page
+          .click()
+          // Wait for hash-link-back-to-same-page to load
+          .waitForElementByCss('#to-other-page')
+
+        await check(
+          async () => {
+            const val = await browser.eval('window.pageYOffset')
+            return val.toString()
+          },
+          (0).toString(),
+          true,
+          // Try maximum of 15 seconds
+          15
+        )
+      })
+    })
+
     describe('not-found', () => {
       it('should trigger not-found in a server component', async () => {
         const browser = await next.browser('/not-found/servercomponent')