Router.Refresh() Eagerly Refetches In-Viewport Link Prefetches In Next.Js V16 Causing Massive ISR Write Amplification

Next.js v16's client-side router reducer now treats prefetch cache invalidation on router.refresh() as an eager revalidation by default, actively refetching every in-viewport prefetched route segment instead of lazily marking entries as stale. This is an undocumented behavioral change likely introduced during the per-segment prefetching refactor and lacks an opt-out configuration.

1. Build and start the v16 app with `next build && next start` (or use the deployed Vercel instance). 2. Open the controller page and separate /blog tabs; scroll blog so several post <Link>s are in viewport and prefetched. 3. Open DevTools Network, filter prefetch requests, add x-vercel-cache column. 4. Click 'Call revalidateTag()' on controller; it broadcasts router.refresh() to blog tabs. 5. Observe that every in-viewport <Link> immediately fires new prefetch requests with x-vercel-cache: REVALIDATED, unlike v15 where no extra prefetches occur.

This workaround disables automatic viewport prefetching, which may increase navigation latency, especially on mobile or slow connections. Manual prefetch on hover/touch may still generate requests, though far fewer than viewport-based eager refetch. The API for router.prefetch and Link props may change in future Next.js versions; test thoroughly after upgrades. Rolling back this change restores the original eager behavior, so monitor ISR writes and navigation metrics before and after.