(8/19) Hydrate export dynamic fallbacks on initial load

[feedthejim]

Stack position

Part 8 of 19 in the output export dynamic fallback stack.

Previous PR: #93568 (7/19)
Next PR: #93561 (9/19)

Context

The stack is split so build-time output can land behind experimental.outputExportDynamicFallbacks before the client router behavior is enabled. The target behavior is for output: 'export' apps with parameterized App Router routes to emit fallback HTML/RSC artifacts that a later client can resolve for hard loads and soft navigations.

This PR scope: Client bootstrap for hard loads of fallback documents.

At this point the build can emit fallback documents and RSC data, but a browser hard-load still lands on _fallback.html. This PR makes initial load fetch the route-specific fallback payload and render the requested route.

What changed

• Adds the fallback bootstrap path for _fallback.html initial loads.
• Fetches route-specific fallback or not-found RSC instead of hydrating stale _fallback.html data.
• Uses createRoot for fallback bootstrap and removes the temporary hidden style after the first commit.
• Removes the hidden original-URL session/global side channel. The requested URL is read directly from window.location.

Deliberately not included

• Does not make soft navigations work.
• Does not add segment-cache fallback keying.
• Does not rely on the removed __NEXT_EXPORT_ORIGINAL_URL mechanism.

Verification

After restacking and autosquashing, I verified the full stack with:

• pnpm --filter=next build
• pnpm testonly packages/next/src/lib/output-export-dynamic-fallback.test.ts packages/next/src/lib/output-export-fallback-routes.test.ts packages/next/src/export/helpers/output-export-fallback.test.ts packages/next/src/server/request/fallback-params.test.ts
• pnpm testonly packages/next/src/client/output-export-fallback.test.ts packages/next/src/client/components/router-reducer/fetch-server-response.test.ts packages/next/src/client/components/router-reducer/create-initial-router-state.test.ts packages/next/src/client/components/segment-cache/cache.test.ts packages/next/src/client/components/segment-cache/vary-path.test.ts
• pnpm testonly packages/next/src/client/components/router-reducer/compute-changed-path.test.ts packages/next/src/client/flight-data-helpers.test.ts packages/next/src/server/app-render/postponed-state.test.ts
• pnpm test-start-turbo test/e2e/app-dir-export/test/dynamic-fallback-cache-components.test.ts test/e2e/app-dir-export/test/dynamic-fallback-route-shapes-cache-components.test.ts test/e2e/app-dir-export/test/dynamic-fallback-cache-components-basepath.test.ts test/e2e/app-dir-export/test/dynamic-fallback-optimizations-cache-components.test.ts test/e2e/app-dir-export/test/dynamic-fallback-known-params-cache-components.test.ts test/e2e/app-dir-export/test/dynamic-fallback-conflict.test.ts
• pnpm test-start-turbo test/e2e/app-dir/output-export-dynamic-fallbacks/output-export-dynamic-fallbacks.test.ts test/e2e/app-dir-export/test/output-export-server-io.test.ts test/e2e/app-dir-export/test/output-export-server-io-use-cache.test.ts test/e2e/app-dir-export/test/dynamic-fallback-server-params.test.ts

[github-actions]

Failing test suites

Commit: 7631b89 | About building and testing Next.js

pnpm test-start test/e2e/app-dir/app-prefetch/prefetching.test.ts (job)

• app dir - prefetching > should immediately render the loading state for a dynamic segment when fetched from higher up in the tree (DD)

Expand output

● app dir - prefetching › should immediately render the loading state for a dynamic segment when fetched from higher up in the tree

thrown: "Exceeded timeout of 60000 ms for a test.
Add a timeout value to this test to increase the timeout, if this is a long-running test. See https://jestjs.io/docs/api#testname-fn-timeout."

  50 |       }
  51 |
> 52 |       const result = Reflect.apply(target, thisArg, args)
     |                              ^
  53 |       return typeof result === 'function' ? wrapJestTestFn(result) : result
  54 |     },
  55 |     get(target, prop, receiver) {

  at Object.apply (lib/e2e-utils/index.ts:52:30)
  at it (e2e/app-dir/app-prefetch/prefetching.test.ts:314:3)
  at Object.describe (e2e/app-dir/app-prefetch/prefetching.test.ts:11:1)