Sprint 129 — Post-login Language Toggle Regression Fix

Background

User report: "Language toggle button does not work after login." Toggle works normally in non-logged-in state (landing/other public pages), but symptom only reproduces on protected routes like dashboard/admin after login. Suspected regression since Sprint 126 Wave P0 (lib/api/client.ts locale-aware redirect).

Delegated Phase 1 static analysis to Herald → Established 5 hypotheses (H1~H5), then verified against entire codebase. H3 (Secondary Bug) + H1 (partial confirmed) that can be confirmed by static analysis → immediately fixed in Wave A. H4 (Primary Suspect) requiring runtime verification → deferred to Wave B.

Processing Status

Phase 1 — Hypothesis Verification Matrix (Herald Static Analysis)

Herald inbox: ~/.claude/oracle/inbox/herald-task-20260425-122835-21417.md

Decision principle: Static analysis confirmed → Wave A immediate fix, runtime verification needed → Wave B defer.

Wave A — H3 + H1 Immediate Fix

Assigned: Herald (implementation), Critic (cross-review)

Herald inbox: ~/.claude/oracle/inbox/herald-task-20260425-125011-22404.md

A-1 — H3 fix: AppLayout.tsx usePathname import replacement (commit a4fa7c9)

Problem: AppLayout.tsx:17 imports usePathname from next/navigation. next/navigation usePathname returns full path including locale prefix (/en, /ko). isActive discrimination logic expecting locale-stripped pathname (/dashboard) always mismatches → sidebar nav highlight disappears in English locale.

Fix: Replace next/navigation → @/i18n/navigation (next-intl wrapper). @/i18n/navigation's usePathname returns path with locale prefix removed.

- import { usePathname, useRouter } from 'next/navigation';
+ import { usePathname } from '@/i18n/navigation';
+ import { useRouter } from 'next/navigation';

Impact: isActive('/dashboard') correctly evaluates to true in English locale (/en/dashboard) → sidebar highlight restored.

A-2 — H1 fix: LanguageSwitcher 4 locations Suspense boundary added (commit bddb225)

Problem: LanguageSwitcher calls useSearchParams() internally. In Next.js 14 App Router, Suspense boundaries without useSearchParams() may interrupt SSR rendering. May cause toggle mount failure due to hydration instability when entering protected routes after login.

Fix points 4 locations:

admin layout requires no changes — already resolved by AppLayout internal Suspense.

// Before
<LanguageSwitcher />

// After
<Suspense fallback={null}>
  <LanguageSwitcher />
</Suspense>

A-3 — 8 New Regression Tests (commit 5f42157)

AppLayout.test.tsx — isActive locale-aware 4 items:

• ko locale: /ko/dashboard pathname → isActive('/dashboard') true
• en locale: /en/dashboard pathname → isActive('/dashboard') true
• locale-stripped /dashboard → isActive('/dashboard') true (preserve existing behavior)
• Non-matching path → isActive('/settings') false

LanguageSwitcher.test.tsx — Suspense + EN locale 4 items:

• Rendering success when mounted within Suspense boundary
• Normal rendering in EN locale
• aria-label locale-aware verification
• Current locale display accuracy

Jest result: 1400 → 1408 (+8 new) PASS.

Change Summary

5 files, +167/-15

Critic 1st Cross-Review

Critic inbox: ~/.claude/oracle/inbox/critic-task-20260425-130033-23349.md

Codex verdict original:

"The changes appear to correctly wrap the existing LanguageSwitcher call sites in Suspense and switch AppLayout to the locale-aware pathname hook without introducing any clear regressions. I did not find a discrete, actionable bug in the diff."

Session ID: 019dc2cc-4408-7803-af83-7094fe8c85e4

Summary: ✅ Ready to merge (No Critical/High, Medium is dead code so not a blocker)

Wave B Deferral Decision — H4 (Primary Suspect)

Deferral Reason

Phase 1 static analysis identified the possibility that client.ts:113-117's window.location.href hard override conflicts with 401 redirect occurring during locale-switch, but:

• Cannot confirm actual collision path by static analysis (depends on component mount order + SWR revalidation timing)
• Possibility exists that H3/H1 fix alone resolves the post-login toggle symptom

Decision Criteria

After Wave A merge:

• Toggle works normally → H4 is a false alarm, concluded, Wave B unnecessary
• Toggle still not working → Start Wave B:
  • Option A: isLocaleTransitioning flag — temporarily suppress 401 redirect during locale-switch
  • Option B: SWR onError per-component handling — router branching instead of global window.location.href

Planned Implementation Location

Sprint 129 follow-up Wave B or Sprint 130 new item (decided based on user reproduction results).

Verification Results

• npx tsc --noEmit: passed
• npx next lint: 0 new warnings
• npx jest: 131 suites, 1408 tests passing (+8 new)
• Critic 1st codex passed (session 019dc2cc)

Agent Collaboration

New Patterns / Lessons

1. usePathname import always prefers locale-aware version

next/navigation's usePathname returns full path including locale prefix (/en, /ko). Use @/i18n/navigation's usePathname in all places that need locale-stripped pathname.

// ❌ includes locale prefix — isActive('/dashboard') mismatch
import { usePathname } from 'next/navigation';

// ✅ removes locale prefix — isActive('/dashboard') correct evaluation
import { usePathname } from '@/i18n/navigation';

This pattern is registered as a Sensei education session candidate (@/i18n/navigation vs next/navigation mixed use detection).

2. Components using useSearchParams must be wrapped in Suspense at call site

Components that internally call useSearchParams() (LanguageSwitcher) must be wrapped in <Suspense fallback={null}> at each call site in the rendering tree to ensure SSR hydration stability. Self-wrapping Suspense inside the component is insufficient — boundary must be specified at every mount point.

3. Phase 1 static analysis → hypothesis classification → Wave-unit progression pattern

Phase 1 (Herald static analysis)
  └─ Statically confirmable (H3/H1) → Wave A immediate fix
  └─ Runtime verification needed (H4) → Wave B deferred
       └─ Determined by reproduction after Wave A merge

Fixing statically confirmed bugs first and confirming runtime bugs later via user reproduction reduces unnecessary speculative fixes and lowers risk.

4. dead code TopNav — incremental cleanup pattern

Critic identified TopNav:335 Suspense missing as Medium, but "0 imports" makes it non-blocking. Pattern of registering as seed and removing in a separate Wave (or fixing together when going live) rather than immediate deletion. Even when Critic finds dead code, blocker level is downgraded (Medium → sprint seed).

Sprint 130 Seeds (3 items)

Reference