fix(web): preserve locale preference during prefetch

[teamleaderleo]

A Korean-route prefetch can complete after the language picker has selected English and overwrite NEXT_LOCALE=en with ko. The page remains English, but reloads use the stale preference. A browser trace reproduced this ordering: the prefetch requested /ko/blog/cmux-vault with the new English cookie, then returned Set-Cookie: NEXT_LOCALE=ko.

Suppress next-intl's cookie publication for all router-prefetch variants (including static 1, runtime 2, and shell 3) and purpose: prefetch, preserving its locale routing and request headers. Rebuild the response without the cookie headers so later cookie writes cannot resurrect the preference from an internal cookie map. Ordinary document and RSC navigation still persist the destination locale. The language picker and existing browser assertions are unchanged.

Validation: the static/purpose regressions fail before the initial fix, and the runtime/shell regressions fail before the review fix. All 10 focused middleware/link tests, typecheck, and the complexity check pass on the current head. The initial fix also passed all four existing production-browser locale cases (Korean/Japanese/Arabic roundtrips and nested-route reload), 20 further nested-route repetitions, and the complexity gate across all tracked web sources. Fresh hosted browser/CI results are required for the review follow-up. The original browser failure was preserved (1 failure in 10 baseline nested-route repetitions), rather than waived as unrelated CI noise.

Fixes #13246.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 692f2c0 and 65b4b0b.

📒 Files selected for processing (2)

• web/proxy.ts
• web/tests/locale-prefetch-cookie.test.ts

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[github-actions]

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the previously missed runtime-prefetch variant is handled and no new actionable failure remains.

Summary

The PR prevents delayed locale-route prefetch responses from overwriting a newer explicit locale preference.

• Detects all next-router-prefetch variants as well as purpose: prefetch.
• Rebuilds prefetch responses without locale cookie-publication headers while preserving locale routing metadata.
• Adds deterministic coverage for prefetch header values 1, 2, and 3, plus a real-navigation control.
• The previous finding is fully addressed: runtime prefetch value 2 is now recognized and tested.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Localized request] --> B[next-intl middleware]
    B --> C{Prefetch header present?}
    C -- Yes --> D[Remove cookie publication headers]
    D --> E[Return routing response without changing preference]
    C -- No --> F[Return normal response]
    F --> G[Persist destination locale]

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