[oBusk]

Adopts Partial Prefetching from Next.js 16.4. With it on, a default <Link> prefetches the route's static App Shell, which is shared by every link to that route.

A hard requirement for this PR is zero increase in server CPU, so it only uses prefetches that are served from build output. It does not use <Link prefetch={true}>, router.prefetch(..., { kind: "full" }) or prefetch() stages, because those render on the server once per link.

Changes

• Enable partialPrefetching: Next 16.4 warns when cacheComponents is on without it set, and it becomes the default in Next 17.
• Skeleton fallback for [...parts]: the shell used to be an empty <Suspense>. It now has a skeleton shaped like the top of the diff page, so clicking a diff link paints immediately.
• Separate catalog route: Next serves one App Shell per route, so catalog pages move to an internal /-/catalog/[...parts] route with a catalog skeleton as their shell. Routing follows one rule:
  • A rewrite in next.config.ts sends canonical catalog URLs (/react, /@types/node) to the catalog route. The patterns live in src/lib/catalogRoutes.ts, and tests run them through Next's own path matcher.
  • Any other URL that parses as a catalog page (/react@*, /@types%2Fnode, /%40types/node) reaches [...parts], which redirects it to the canonical URL. A test checks that every such redirect target matches the rewrite, so the redirect can't loop.
  • The internal /-/catalog/... path redirects to the public URL.
  • The catalog components move into the catalog route's folder.
  • The catalog skeleton lives in the route's loading.tsx.
• One error boundary for all routes: the diff route's error.tsx moves to src/app/error.tsx so the catalog route shares it. / and the /-/about pages now get the same error UI instead of Next's default.
• Remove prefetch={false} from the catalog's suggested diffs: a default prefetch now only fetches the shared static shell.
• ensureStatic = "shell" on the root layout: every route, including future ones, must keep its App Shell static.
• ensureStatic = "navigation" on / and the /-/about pages: the build fails if they become dynamic, even inside <Suspense>. Verified by adding a connection() call inside <Suspense> on /-/about, which failed the build.

Server cost

Measured on the preview deployment, with request logs in the Vercel dashboard:

• Every App Shell prefetch (_tree, _index and page segments) is served from build output (x-vercel-cache: HIT/PRERENDER, x-matched-path: /[...parts].segments/… or /-/catalog/[...parts].segments/…). This holds for unknown packages too, so it never invokes a function. Production already serves its default prefetches the same way.
• Prefetches of / and the /-/about pages come from the ISR cache with no function invocation.
• Clicking a catalog diff link makes one static prefetch and one navigation request, the same number of server renders as before.
• New URLs: the Next docs describe rendering not-yet-built URLs again in the background and caching them per URL. That doesn't happen here. Three visits each to a new diff URL and a new catalog URL made exactly one function invocation per visit, and every visit, including the third, was served the route's shared fallback shell. There were no extra background invocations and no per-URL ISR writes. Each visit costs one invocation, the same as production.
• The redirects and rewrites run in Vercel's routing layer. The [...parts] redirect for non-canonical catalog URLs only runs for requests that already invoked a function before this PR.

Testing

In DevTools, open the Network tab, filter on _rsc, and throttle to Slow 4G.

Check	Production (before)	Preview (expected)
Click a suggested diff on a catalog page (e.g. /is-even)	Stays on the old page until the server responds	URL changes and the diff skeleton shows immediately
Click a home page example	Empty shell, then the diff	Diff skeleton immediately, then the diff
Visit /react or /@types/node directly	Catalog page	Catalog page, with a catalog skeleton while streaming
Visit /react@*, /@types%2Fnode or /%40types/node	Catalog page	Redirects to /react or /@types/node
Visit /-/catalog/react	n/a	Redirects to /react
Toggle Split/Unified on a diff page	No prefetch	No prefetch (unchanged)

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Project	Deployment	Actions	Updated
npm-diff	Ready	Preview	Nov 7, 2026 12:24pm UTC