fix redundant fetching of inactive default segments

packages/next/src/server/app-render/create-component-tree.tsx

@@ -15,6 +15,7 @@ import { getTracer } from '../lib/trace/tracer'
 import { NextNodeServerSpan } from '../lib/trace/constants'
 import { StaticGenBailoutError } from '../../client/components/static-generation-bailout'
 import type { LoadingModuleData } from '../../shared/lib/app-router-context.shared-runtime'
+import { DEFAULT_SEGMENT_KEY } from '../../shared/lib/segment'
 
 type ComponentTree = {
   seedData: CacheNodeSeedData
@@ -395,6 +396,10 @@ async function createComponentTreeInternal({
           // This behavior predates PPR and is only relevant if the
           // PPR flag is not enabled.
           isPrefetch &&
+          // If we'd be rendering the default parallel route component, we include it in the tree.
+          // Otherwise, the prefetch would cause any parallel routes rendering the default state
+          // to be refetched on navigation, because the server will have cleared them.
+          parallelRoute[0] !== DEFAULT_SEGMENT_KEY &&
           (Loading || !hasLoadingComponentInTree(parallelRoute)) &&
           // The approach with PPR is different — loading.tsx behaves like a
           // regular Suspense boundary and has no special behavior.

test/production/app-dir/parallel-routes-prefetching/app/[letter]/@grid/default.tsx

@@ -0,0 +1,3 @@
+export default function Default() {
+  return null
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/@grid/loading.tsx

@@ -0,0 +1,3 @@
+export default function Loading() {
+  return <h1>Grid Loading</h1>
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/@grid/page.tsx

@@ -0,0 +1,33 @@
+import Link from 'next/link'
+
+const alphabet = Array.from('abcdefghijklmnopqrstuvwxyz')
+
+function getShuffledAlphabet() {
+  return alphabet
+    .map((value) => ({ value, sort: Math.random() }))
+    .sort((a, b) => a.sort - b.sort)
+    .map(({ value }) => value)
+}
+
+export default async function Page({
+  params: { letter },
+}: {
+  params: { letter: string }
+}) {
+  await new Promise((r) => setTimeout(r, 250))
+
+  return (
+    <>
+      <span id="letter-parallel">{letter}</span>
+      <ul>
+        {getShuffledAlphabet().map((letter) => (
+          <li key={letter}>
+            <Link href={`/${letter}`}>
+              <div>{letter}</div>
+            </Link>
+          </li>
+        ))}
+      </ul>
+    </>
+  )
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/@grid2/default.tsx

@@ -0,0 +1,3 @@
+export default function Default() {
+  return null
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/@grid3/default.tsx

@@ -0,0 +1,3 @@
+export default function Default() {
+  return null
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/@grid4/default.tsx

@@ -0,0 +1,3 @@
+export default function Default() {
+  return null
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/@grid5/default.tsx

@@ -0,0 +1,3 @@
+export default function Default() {
+  return null
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/default.tsx

@@ -0,0 +1,3 @@
+export default function Default() {
+  return null
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/layout.tsx

@@ -0,0 +1,26 @@
+import { ReactNode } from 'react'
+
+type LetterLayoutProps = {
+  params: { letter: string }
+  children: ReactNode
+  grid: ReactNode
+  grid2: ReactNode
+  grid3: ReactNode
+  grid4: ReactNode
+  grid5: ReactNode
+}
+
+export default function LetterLayout(props: LetterLayoutProps) {
+  return (
+    <div className="bg-gray-100">
+      {props.children}
+      <div className="bg-gray-400 p-4">
+        {props.grid}
+        {props.grid2}
+        {props.grid3}
+        {props.grid4}
+        {props.grid5}
+      </div>
+    </div>
+  )
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/loading.tsx

@@ -0,0 +1,3 @@
+export default function Loading() {
+  return <h1>Loading</h1>
+}

test/production/app-dir/parallel-routes-prefetching/app/[letter]/page.tsx

@@ -0,0 +1,8 @@
+export default async function LetterPage({
+  params: { letter },
+}: {
+  params: { letter: string }
+}) {
+  await new Promise((r) => setTimeout(r, 250))
+  return <h1 id="letter-page">{letter}</h1>
+}

test/production/app-dir/parallel-routes-prefetching/app/layout.tsx

@@ -0,0 +1,7 @@
+export default function Root({ children }: { children: React.ReactNode }) {
+  return (
+    <html>
+      <body>{children}</body>
+    </html>
+  )
+}

test/production/app-dir/parallel-routes-prefetching/app/page.tsx

@@ -0,0 +1,5 @@
+import Link from 'next/link'
+
+export default function Page() {
+  return <Link href="/a">Go to a letter</Link>
+}

test/production/app-dir/parallel-routes-prefetching/next.config.js

@@ -0,0 +1,6 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {}
+
+module.exports = nextConfig

test/production/app-dir/parallel-routes-prefetching/parallel-routes-prefetching.test.ts

@@ -0,0 +1,58 @@
+import { nextTestSetup } from 'e2e-utils'
+import { retry } from 'next-test-utils'
+
+describe('parallel-routes-prefetching', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('should not re-fetch data for the __DEFAULT__ segments when navigating within a dynamic segment', async () => {
+    let prefetchRequests = []
+    let rscRequests = []
+    const browser = await next.browser('/a', {
+      beforePageLoad(page) {
+        page.on('request', (request) => {
+          return request.allHeaders().then((headers) => {
+            if (headers['Next-Router-Prefetch'.toLowerCase()] === '1') {
+              prefetchRequests.push(request.url())
+            } else if (headers['RSC'.toLowerCase()] === '1') {
+              rscRequests.push(request.url())
+            }
+          })
+        })
+      },
+    })
+
+    expect(await browser.elementById('letter-page').text()).toBe('a')
+    expect(await browser.elementById('letter-parallel').text()).toBe('a')
+
+    await retry(() => {
+      // we should see 25 prefetch requests for each letter of the alphabet (excluding the current)
+      expect(prefetchRequests).toHaveLength(25)
+      // and 0 RSC requests since we're not navigating to a different dynamic segment
+      expect(rscRequests).toHaveLength(0)
+    })
+
+    prefetchRequests = []
+
+    await browser.elementByCss('[href="/b"]').click()
+
+    await retry(() => {
+      // Navigating to a new dynamic segment should trigger any additional prefetch requests
+      expect(prefetchRequests).toHaveLength(0)
+
+      // but we except 2 RSC requests for the new page:
+      // - one for the `children` slot
+      // - one for the active parallel slot
+      // TODO: It would be nice to achieve this in a single request, but it currently isn't possible,
+      // because the router will see missing data for both slots simultaneously and kick off two separate requests.
+      // PPR doesn't have this issue because it doesn't perform the "lazy fetch" in layout-router.
+      expect(rscRequests).toHaveLength(
+        process.env.__NEXT_EXPERIMENTAL_PPR ? 1 : 2
+      )
+    })
+
+    expect(await browser.elementById('letter-page').text()).toBe('b')
+    expect(await browser.elementById('letter-parallel').text()).toBe('b')
+  })
+})