test: cover blocking not-found status stability

docs/01-app/02-guides/migrating-to-cache-components.mdx

@@ -601,6 +601,8 @@ Paths you don't return are still served. Next.js prerenders a static shell for t
 
 Params not returned by `generateStaticParams` are rendered on request. If you used `dynamicParams: false` to reject them, call [`notFound()`](/docs/app/api-reference/functions/not-found) in the page when the param doesn't resolve to real data.
 
+If this check runs after the Cache Components [static shell](/docs/app/glossary#static-shell) starts streaming, the response keeps its `200` status and Next.js adds a `noindex` meta tag. If you require an HTTP `404`, validate the path before the response streams, such as in [`proxy`](/docs/app/api-reference/file-conventions/proxy). See [Status Codes](/docs/app/api-reference/file-conventions/loading#status-codes).
+
 ### Await `params` inside `<Suspense>`
 
 To produce the static shell, pass the `params` promise into a [`<Suspense>`](/docs/app/api-reference/file-conventions/loading) boundary instead of awaiting it at the top of the component, so unknown params can still prerender.

test/e2e/app-dir/partial-fallback-shell-upgrade/fixtures/partial-prefetching-disabled/app/blocking/[[...slug]]/page.tsx

@@ -0,0 +1,28 @@
+import { notFound } from 'next/navigation'
+
+const prerenderedSlugs = ['known']
+const runtimeSlugs = ['runtime']
+
+export const instant = false
+
+export function generateStaticParams() {
+  return prerenderedSlugs.map((slug) => ({ slug: [slug] }))
+}
+
+export default async function Page({
+  params,
+}: {
+  params: Promise<{ slug?: string[] }>
+}) {
+  const { slug } = await params
+  const pathname = slug?.join('/') ?? ''
+
+  if (
+    !prerenderedSlugs.includes(pathname) &&
+    !runtimeSlugs.includes(pathname)
+  ) {
+    notFound()
+  }
+
+  return <p id="blocking-slug">{pathname}</p>
+}

test/e2e/app-dir/partial-fallback-shell-upgrade/partial-fallback-shell-upgrade.test.ts

@@ -240,4 +240,25 @@ describe('partial-fallback-shell-upgrade - partialPrefetching disabled', () => {
       'generic shell should remain shared without partialPrefetching'
     )
   })
+
+  it('keeps blocking not-found responses stable after the first request', async () => {
+    for (let attempt = 0; attempt < 2; attempt++) {
+      const response = await next.fetch('/blocking/missing')
+      const html = await response.text()
+
+      expect(response.status).toBe(404)
+      expect(html).toContain('<meta name="robots" content="noindex"')
+    }
+  })
+
+  it('serves valid ungenerated params before and after the first request', async () => {
+    for (let attempt = 0; attempt < 2; attempt++) {
+      const response = await next.fetch('/blocking/runtime')
+
+      expect(response.status).toBe(200)
+      expect(await response.text()).toContain(
+        '<p id="blocking-slug">runtime</p>'
+      )
+    }
+  })
 })