Cached Navigations: Cache static stage of partially static initial HTML

[unstubbable]

When a partially static page is loaded via initial HTML (PPR resume), the RSC payload now includes the l (static stage byte length) field, enabling the client router to extract and cache the static stage during hydration. Subsequent navigations to the same page serve the cached static content instantly while streaming in dynamic content.

On the server, the resume path in renderToStream now uses staged rendering (mirroring generateStagedDynamicFlightRenderResult) when Cache Components is enabled. A StagedRenderingController separates the static and dynamic stages, and countStaticStageBytes resolves the byte length promise that Flight serializes into the stream as l. getRSCPayload accepts staleTimeIterable and staticStageByteLengthPromise options to wire s and l into the InitialRSCPayload.

On the client, app-index.tsx tees the inlined Flight stream when Cache Components is enabled. One copy goes to React for decoding, the other is passed to createInitialRouterState. When l is present (partially static), the clone is truncated at the byte boundary via the new decodeStaticStage helper and written into the segment cache via processStaticStageResponse and writeStaticStageResponseIntoCache. When l is absent but s is present (fully static), the existing writeInitialSeedDataIntoCache path is used unchanged.

createInitialRouterState now accepts a single initialRSCPayload: InitialRSCPayload prop instead of individual destructured fields. resolveStaticStageData, processStaticStageResponse, writeStaticStageResponseIntoCache, and writeDynamicRenderResponseIntoCache are widened to accept NavigationFlightResponse | InitialRSCPayload.

[unstubbable]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Cached Navigations: Cache static stage of partially static initial HTML #90539 👈 (View in Graphite)
• Cached Navigations: Cache visited fully static pages in the segment cache #90306
• Cached Navigations: Serve cached segments instantly on repeat visits #90223
• Fix runtime prefetch isPartial byte stripping when deployed #90272
• canary

This stack of pull requests is managed by Graphite. Learn more about stacking.

[nextjs-bot]

Failing test suites

Commit: d37c0ed | About building and testing Next.js

pnpm test-dev-turbo test/development/dev-indicator/dev-rendering-indicator.test.ts (turbopack) (job)

• Dev Rendering Indicator > Shows build indicator when page is built from modifying (DD)

Expand output

● Dev Rendering Indicator › Shows build indicator when page is built from modifying

expect(received).toEqual(expected) // deep equality

- Expected  - 1
+ Received  + 1

  Object {
-   "showedRenderingIndicator": true,
+   "showedRenderingIndicator": false,
  }

  42 |
  43 |     const showedRenderingIndicator = await browser.eval('window.showedBuilder')
> 44 |     expect({ showedRenderingIndicator }).toEqual({
     |                                          ^
  45 |       showedRenderingIndicator: true,
  46 |     })
  47 |   })

  at Object.toEqual (development/dev-indicator/dev-rendering-indicator.test.ts:44:42)