* perf(app-router): defer layout, template and boundary modules to first request
The generated RSC entry statically imported every route's layout, template,
loading/error/not-found/forbidden/unauthorized boundary and parallel-slot
module at the top level. On a cold isolate — and on the dev server's first
request — that eagerly evaluated the whole app's segment tree, including
routes never visited in that lifetime. Page and route-handler modules were
already lazy; the surrounding segment modules were not.
Emit those modules as lazy `() => import()` loaders paired with `null`
placeholders in the manifest, and hydrate them onto the route's synchronous
fields in `ensureAppRouteModulesLoaded` before any read (extended from
page/handler to layouts, templates, boundaries and parallel-slot modules).
Intercept layouts hydrate via the dedicated `loadAppInterceptLayouts` because
they live on the match object, not the route. Every reader already runs behind
`ensureAppRouteModulesLoaded` (handler dispatch and `buildPageElements`) or the
intercept resolvers, so no synchronous module field is read before hydration.
The shared intercept `interceptLayouts` arrays keep their `readonly` public
types; the loader is their sole writer and fills slots through one
boundary-local cast, so route metadata stays immutable to callers.
Dev cold start is unchanged on page-heavy apps — pages were already lazy — but
on a boundary-heavy app (80 independent sections, each with
layout+template+error+loading+not-found) first-request module compile drops
704ms -> 534ms (~24%) and time-to-first-200 1917ms -> 1665ms. The saving scales
with the number of unrelated segments deferred.
* test(app-router): localize react max header font fixture
* fix(app-router): dedupe lazy intercept module loads
* fix(app-router): share intercept load state across matches
* docs(app-router): explain root layout loader reuse
---------
Co-authored-by: James <james@eli.cx>
* fix(app-router): respect reactMaxHeadersLength for preload Link header (#1552)
* test(app-router): cover reactMaxHeadersLength config resolution
* refactor(app-router): prioritize React preloads in Link header; cover buildAppPageLinkHeader
Address review feedback on #1841:
- Order React preload hints before font preloads so render-critical entries
survive a tight reactMaxHeadersLength cap.
- Rename the buildAppPageHtmlResponse option to linkHeader (it now carries the
combined header, not just fonts).
- Add unit coverage for buildAppPageLinkHeader (cap, 0-disables, ordering,
whole-entry truncation).
Nathan Nguyen