fix(pages-router): collapse doubled basePath in client asset URLs

[NathanDrake2406]

What this changes

Pages Router apps built with a basePath now emit their page and _app
client chunk URLs with the basePath applied exactly once
(/docs/_next/static/_slug_.js) instead of twice
(/docs/docs/_next/static/_slug_.js).

Why

With a basePath (e.g. /docs), the rendered HTML pointed every page and
_app chunk at a doubled path:

<link rel="modulepreload" href="/docs/docs/_next/static/_slug_-….js">
<script type="module" src="/docs/docs/_next/static/_slug_-….js"></script>

"__vinext":{"pageModuleUrl":"/docs/docs/_next/static/_slug_-….js", ...}

Those URLs 404, so the client bundle never loads, the page never hydrates,
next.router never initializes, and any client-side navigation silently does
nothing. <div id="__next"> stays empty.

This surfaced as two failing cases in the upstream Next.js suite
test/e2e/basepath/error-pages.test.ts:

• basePath › client-side navigation › should navigate to /404 correctly client-side
• basePath › client-side navigation › should navigate to /_error correctly client-side

Both call next.router.push("/404", "/slug-2") and time out waiting for the
404 page, because the router that would handle the push never came online.

Approach

Root cause is a base-applied-twice interaction in the build:

1. Vite+ bakes the configured base into emitted chunk fileNames on disk
   (dist/client/docs/_next/static/...). vinext relies on that layout so the
   Cloudflare ASSETS binding serves /docs/_next/static/... directly.
2. Vite then prepends base again when it writes ssr-manifest.json,
   producing docs/docs/_next/static/... entries.
3. augmentSsrManifestFromBundle
   preserved those doubled entries verbatim (it only stripped a leading slash),
   while its own bundle backfill keyed off chunk.fileName and produced the
   correct single-prefix path. The merged manifest held both, and
   resolveClientModuleUrl returned the first (doubled) entry.

The on-disk chunk fileName carries the basePath exactly once, so the public URL
must too. New collapseDuplicateBase helper collapses an exact
<base>/<base>/ prefix when normalizing the Vite-sourced manifest entries, so
they line up with the single-base paths the backfill already produces. A
correctly single-prefixed entry is left untouched (no over-collapse).

The fix lives at the single boundary that owns the manifest shape, so every
downstream reader (modulepreload tags, __vinext.pageModuleUrl, CSS links) is
corrected at once.

Non-goals

• App Router is unaffected: its bootstrap script URLs come from the RSC
  plugin's Vite runtime base resolution, which already emitted single-base URLs
  even in the broken output.

Validation

• New unit tests on augmentSsrManifestFromBundle:
  • collapses a basePath that Vite duplicated in the SSR manifest (fails on the
    old implementation, passes with the fix)
  • leaves a single basePath prefix untouched (guards against over-collapse)
• Rebuilt the pages-i18n-domains-basepath fixture and confirmed the emitted
  ssr-manifest.json no longer contains any app/app/... entries.
• Ran the upstream deploy suite end to end:
  test/e2e/basepath/error-pages.test.ts went from 2 failed / 6 passed to
  8 passed / 1 skipped.
• vp check and the full tests/build-optimization.test.ts +
  tests/deploy.test.ts suites pass.

Risks / follow-ups

• The collapse only triggers on an exact <base>/<base>/ prefix, so it is a
  no-op for builds where the bundler does not bake base into fileNames (the
  paths are already single-prefixed). No behavior change for apps without a
  basePath.

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commit: aecfec4

[james-elicx]

/bigbonk review

[ask-bonk]

Review: collapse doubled basePath in client asset URLs

Clean, well-scoped fix. The root-cause analysis is accurate and the fix is placed at the single boundary (augmentSsrManifestFromBundle) that owns the SSR manifest shape, so every downstream reader (resolveClientModuleUrl in pages-server-entry.ts:565, modulepreload tags, __vinext.pageModuleUrl, CSS links) is corrected at once.

Verified

• Bug reproduction is real. resolveClientModuleUrl returns the first .js entry in the manifest file list (pages-server-entry.ts:568-573). The old normalizeManifestFile only stripped a leading slash, so the Vite-doubled docs/docs/_next/static/... entry was preserved verbatim and returned first → 404 → no hydration. The regression test would fail on the old implementation.
• Dedup works. The collapsed ssr-manifest entry (docs/_next/static/_slug_.js) and the bundle-backfill entry (manifestFileWithBase(chunk.fileName, base), which leaves the already-baked docs/ prefix untouched per manifest-paths.ts:13) produce the same string, so the Set<string> dedups them. No leftover doubled entry.
• App Router parity / no regression. augmentSsrManifestFromBundle is shared (index.ts:4278), but collapse only triggers on an exact <base>/<base>/ prefix, so it is a no-op for App Router and for builds without a basePath. The over-collapse guard test confirms this.
• Dev/prod parity. The doubling is build-only (dev uses Vite's runtime module resolution, not this static manifest), so the build-time fix is the correct and complete location. No matching prod-server-only path was left unfixed.
• vp check passes on both changed source files; the augmentSsrManifestFromBundle test block passes (8 tests).

One minor note (non-blocking)

The collapse keys on an exact <base>/<base>/ prefix. There is a theoretical over-collapse if a real (post-base-strip) asset path itself begins with the base segment, e.g. base /a with a genuine asset at a/a/x.js. For Next.js this can't happen in practice since asset paths always start with _next/static/, and the doc comment already calls out the exact-prefix-only behavior, so this is fine as-is — just worth being aware of if the helper is ever reused for non-_next assets.

LGTM.

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