Next.js instant navs PR

Next.js has added an `unstable_instant` route setting that checks whether a page can show an immediate static shell during client-side navigation, before slower data finishes streaming in. (nextjs.im) The feature landed in pull request #91334, which GitHub indexes as “Add eval and docs for unstable_instant,” and it is now documented as a development-time and build-time validation step in the App Router. (app.semanticdiff.com, nextjs.org) In plain terms, the shell is the first usable frame of a page — a skeleton, header, or layout that appears before live data arrives. Next.js already uses React Suspense and `loading.js` to stream that fallback UI, and its docs say the fallback can be prefetched so navigation feels immediate. (nextjs.org, nextjs.org) `unstable_instant` does not make every route fast by itself. The docs say it verifies that the route’s caching structure can produce an instant static shell at every shared layout boundary, and it surfaces errors in development or fails the build when a component would block navigation. (nextjs.im) The rules are specific: `unstable_instant` only works when Cache Components are enabled, it cannot be used in Client Components, and components that read cookies or headers are treated as dynamic and must sit behind a Suspense boundary. (nextjs.im) That fits the way Next.js now describes App Router navigation. The framework says layouts and pages are React Server Components by default, and its navigation guide centers on prefetching, prerendering, streaming, and client-side transitions rather than waiting for a full page render before the route swap. (nextjs.org, nextjs.org) The same docs add a new debugging path for these shells. With `experimental.instantNavigationDevToolsToggle` enabled, developers can open Next.js DevTools, select “Instant Navs,” reload into the initial static UI, or force link clicks to show the prefetched shell instead of the completed result. (nextjs.im) Next.js also documents a Playwright helper named `instant` for testing the frozen shell before dynamic content appears, plus one current caveat: the DevTools use a shared cookie on `localhost`, so multiple local projects can interfere with each other until the feature is stabilized. (nextjs.im) The broader argument here is about what “instant” should mean in modern frameworks. Next.js is moving toward validated, testable shells that preserve Suspense behavior, while Svelte is separately pitching a compiler-first model aimed at doing less work in the browser and supporting newer agent-oriented tooling patterns. (nextjs.im, svelte.dev, github.com) For Next.js teams, the immediate change is practical: routes can now be checked for whether the first frame appears instantly, and the framework has started adding tools to inspect that frame instead of guessing what users saw. (nextjs.im, nextjs.org)