fix(stats): address Claude review follow-ups

backlog/tasks/task-205 - Address-PR-19-Claude-frontend-review-follow-ups.md

@@ -0,0 +1,62 @@
+---
+id: TASK-205
+title: 'Address PR #19 Claude frontend review follow-ups'
+status: Done
+assignee:
+  - codex
+created_date: '2026-03-20 02:41'
+updated_date: '2026-03-20 02:46'
+labels: []
+milestone: m-1
+dependencies: []
+references:
+  - stats/src/components/vocabulary/VocabularyTab.tsx
+  - stats/src/hooks/useSessions.ts
+  - stats/src/hooks/useTrends.ts
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Assess Claude's latest PR #19 review, apply any valid frontend fixes from that review batch, and verify the stats dashboard behavior stays unchanged aside from the targeted performance and error-handling improvements.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 VocabularyTab avoids recomputing expensive known-word and summary aggregates on unrelated rerenders while preserving current displayed values.
+- [x] #2 useSessions and useSessionDetail normalize rejected values into stable string errors without throwing from the catch handler.
+- [x] #3 Targeted tests cover the addressed review items and pass locally.
+- [x] #4 Any user-facing docs remain accurate after the changes.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add focused tests that fail on the current branch for the two valid Claude findings: render-time aggregate recomputation in VocabularyTab and unsafe non-Error rejection handling in useSessions/useSessionDetail.
+2. Update VocabularyTab to memoize the expensive summary and known-word aggregate calculations off the existing filteredWords/kanji/knownWords inputs without changing rendered values.
+3. Normalize hook error handling to convert unknown rejection values into stable strings, matching the existing useTrends pattern.
+4. Run the targeted stats/frontend test lane, verify no docs changes are needed, and record results in task notes.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+Validated Claude's latest PR #19 review comment from 2026-03-20 and narrowed it to two valid frontend follow-ups: memoized VocabularyTab aggregates and non-Error-safe session hook error handling.
+
+Added focused regression tests in stats/src/lib/vocabulary-tab.test.ts and stats/src/hooks/useSessions.test.ts before patching the implementation.
+
+Verification: `cd stats && bun test src/lib/vocabulary-tab.test.ts src/hooks/useSessions.test.ts` passed; `bun run format:check:stats` passed.
+
+Project-native verifier (`.agents/skills/subminer-change-verification/scripts/verify_subminer_change.sh --lane core ...`) passed root `bun run typecheck` and failed at `bun run test:fast` due an unrelated existing failure in `scripts/update-aur-package.test.ts` (`mapfile: command not found`). Artifact: `.tmp/skill-verification/subminer-verify-20260319-194525-vxVD9V`.
+
+No user-facing docs changes were needed because the fixes only affect render-time memoization and error normalization.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Assessed Claude's latest PR #19 review and applied the two valid follow-ups. `stats/src/components/vocabulary/VocabularyTab.tsx` now memoizes `buildVocabularySummary(filteredWords, kanji)` and the known-word count so unrelated rerenders do not rescan the filtered vocabulary list. `stats/src/hooks/useSessions.ts` now exports a small `toErrorMessage` helper and uses it in both `useSessions` and `useSessionDetail`, preventing `.catch()` handlers from throwing when a promise rejects with a non-`Error` value.
+
+Added targeted regressions in `stats/src/lib/vocabulary-tab.test.ts` and `stats/src/hooks/useSessions.test.ts` to lock in the memoization shape and error normalization behavior. Verification passed for `cd stats && bun test src/lib/vocabulary-tab.test.ts src/hooks/useSessions.test.ts` and `bun run format:check:stats`. The repo-native verification wrapper for the classified `core` lane also passed root `bun run typecheck`, but `bun run test:fast` is currently blocked by an unrelated existing failure in `scripts/update-aur-package.test.ts` (`mapfile: command not found`); artifacts are recorded under `.tmp/skill-verification/subminer-verify-20260319-194525-vxVD9V`.
+<!-- SECTION:FINAL_SUMMARY:END -->

stats/src/components/vocabulary/VocabularyTab.tsx

@@ -47,6 +47,19 @@ export function VocabularyTab({
     if (excluded.length > 0) result = result.filter((w) => !isExcluded(w));
     return result;
   }, [words, hideNames, excluded, isExcluded]);
+  const summary = useMemo(
+    () => buildVocabularySummary(filteredWords, kanji),
+    [filteredWords, kanji],
+  );
+  const knownWordCount = useMemo(() => {
+    if (knownWords.size === 0) return 0;
+
+    let count = 0;
+    for (const w of filteredWords) {
+      if (knownWords.has(w.headword)) count += 1;
+    }
+    return count;
+  }, [filteredWords, knownWords]);
 
   if (loading) {
     return (
@@ -63,15 +76,6 @@ export function VocabularyTab({
     );
   }
 
-  const summary = buildVocabularySummary(filteredWords, kanji);
-
-  let knownWordCount = 0;
-  if (knownWords.size > 0) {
-    for (const w of filteredWords) {
-      if (knownWords.has(w.headword)) knownWordCount++;
-    }
-  }
-
   const handleSelectWord = (entry: VocabularyEntry): void => {
     onOpenWordDetail?.(entry.wordId);
   };

stats/src/hooks/useSessions.test.ts

@@ -0,0 +1,20 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import test from 'node:test';
+import { fileURLToPath } from 'node:url';
+import { toErrorMessage } from './useSessions';
+
+const USE_SESSIONS_PATH = fileURLToPath(new URL('./useSessions.ts', import.meta.url));
+
+test('toErrorMessage normalizes Error and non-Error rejections', () => {
+  assert.equal(toErrorMessage(new Error('network down')), 'network down');
+  assert.equal(toErrorMessage('bad gateway'), 'bad gateway');
+  assert.equal(toErrorMessage(503), '503');
+});
+
+test('useSessions and useSessionDetail route catch handlers through toErrorMessage', () => {
+  const source = fs.readFileSync(USE_SESSIONS_PATH, 'utf8');
+  const matches = source.match(/setError\(toErrorMessage\(err\)\)/g);
+
+  assert.equal(matches?.length, 2);
+});

stats/src/hooks/useSessions.ts

@@ -3,6 +3,10 @@ import { getStatsClient } from './useStatsApi';
 import { SESSION_CHART_EVENT_TYPES } from '../lib/session-events';
 import type { SessionSummary, SessionTimelinePoint, SessionEvent } from '../types/stats';
 
+export function toErrorMessage(err: unknown): string {
+  return err instanceof Error ? err.message : String(err);
+}
+
 export function useSessions(limit = 50) {
   const [sessions, setSessions] = useState<SessionSummary[]>([]);
   const [loading, setLoading] = useState(true);
@@ -21,7 +25,7 @@ export function useSessions(limit = 50) {
       })
       .catch((err) => {
         if (cancelled) return;
-        setError(err.message);
+        setError(toErrorMessage(err));
       })
       .finally(() => {
         if (cancelled) return;
@@ -77,7 +81,7 @@ export function useSessionDetail(sessionId: number | null) {
       })
       .catch((err) => {
         if (cancelled) return;
-        setError(err.message);
+        setError(toErrorMessage(err));
       })
       .finally(() => {
         if (cancelled) return;

stats/src/lib/vocabulary-tab.test.ts

@@ -1,11 +1,10 @@
 import assert from 'node:assert/strict';
 import fs from 'node:fs';
-import path from 'node:path';
 import test from 'node:test';
+import { fileURLToPath } from 'node:url';
 
-const VOCABULARY_TAB_PATH = path.resolve(
-  import.meta.dir,
-  '../components/vocabulary/VocabularyTab.tsx',
+const VOCABULARY_TAB_PATH = fileURLToPath(
+  new URL('../components/vocabulary/VocabularyTab.tsx', import.meta.url),
 );
 
 test('VocabularyTab declares all hooks before loading and error early returns', () => {
@@ -20,3 +19,16 @@ test('VocabularyTab declares all hooks before loading and error early returns',
 
   assert.deepEqual(hooksAfterLoadingGuard ?? [], []);
 });
+
+test('VocabularyTab memoizes summary and known-word aggregate calculations', () => {
+  const source = fs.readFileSync(VOCABULARY_TAB_PATH, 'utf8');
+
+  assert.match(
+    source,
+    /const summary = useMemo\([\s\S]*buildVocabularySummary\(filteredWords, kanji\)[\s\S]*\[filteredWords, kanji\][\s\S]*\);/,
+  );
+  assert.match(
+    source,
+    /const knownWordCount = useMemo\(\(\) => \{[\s\S]*for \(const w of filteredWords\) \{[\s\S]*knownWords\.has\(w\.headword\)[\s\S]*\}\s*return count;\s*\}, \[filteredWords, knownWords\]\);/,
+  );
+});