fix: add React Router v7 future flags to BrowserRouter

Add future={{ v7_startTransition: true, v7_relativeSplatPath: true }}
to the production BrowserRouter in src/index.tsx, aligning it with the
test-environment MemoryRouter mock in setupTests.ts.

Eliminates 26 React Router Future Flag Warning occurrences from the
test suite.

Author: khoidt

TASK-683-log.md

@@ -735,3 +735,50 @@
   - `TASK-683-issues-summary.md`: P1 status changed from "Mitigation implemented" to "Resolved"; Next Verification Checklist step 1 marked done.
 - Lint gate: ✓ PASSED.
 - TypeScript gate: ✓ PASSED.
+
+### Investigate test diagnostic hotspots warning classes
+
+- Input: `TASK-683-test-diag-hotspots-2026-03-25.md` (5 warning classes, 515 total occurrences).
+- Researched all 5 warning classes across the codebase:
+
+#### 1. React Router Future Flag Warning (26 occurrences)
+
+- Partially fixed: `src/setupTests.ts` already mocks `MemoryRouter` with `v7_startTransition` and `v7_relativeSplatPath` flags.
+- Remaining: production `BrowserRouter` in `src/index.tsx` has no future flags set.
+- Fix: add `future={{ v7_startTransition: true, v7_relativeSplatPath: true }}` to `<Router>` in `src/index.tsx`.
+
+#### 2. `act(...)` warnings (173 occurrences)
+
+- Root cause: `fireEvent` calls without `act()` wrapping; async state updates after unmount.
+- Shared helpers `changeValue()` and `clickNth()` in `src/test-support/utils.ts` lack `act()`.
+- Strategy: fix shared helpers first (high-leverage), then audit individual test files, consider migrating to `userEvent`.
+
+#### 3. `validateDOMNesting` (10 occurrences)
+
+- 4 violations: `<div>` in `<tbody>` — `ErrorBoundary` via `withData` wrapping table content in `src/signs/ui/search/SignsSearch.tsx`.
+- 2+2 violations: `<figure>`/`<figcaption>` in `<p>` — `src/markup/ui/markup.tsx` uses `<p>` container for block-level markup content.
+- 1 violation: `<td>` in `<div>` — `LineLemmasContext.Provider` wrapping `<td>` in `src/transliteration/ui/LineAccumulator.tsx`.
+
+#### 4+5. `controlId ignored on FormLabel/FormControl` (153+153 = 306 occurrences)
+
+- React Bootstrap `^2.9.0` warns when `Form.Group` has `controlId` and children also have explicit `htmlFor`/`id`.
+- Key files: `LemmaInput.tsx`, `DateConverterFormField.tsx`, `CuneiformConverterForm.tsx`, plus many more.
+- Fix: remove redundant `htmlFor`/`id` from children (keep `controlId`), or remove `controlId` if manual management is intended.
+
+#### Prioritized fix plan
+
+1. React Router future flags — 1 file change, eliminates 26 warnings.
+2. controlId cleanup — mechanical, eliminates 306 warnings.
+3. validateDOMNesting — targeted structural fixes, eliminates 10 warnings.
+4. act() warnings — largest effort (phased), eliminates 173 warnings.
+
+- Updated `TASK-683-todo.md` with 4 new fix items.
+
+### Fix #1: React Router future flag warnings (26 occurrences)
+
+- Added `future={{ v7_startTransition: true, v7_relativeSplatPath: true }}` to `<Router>` (BrowserRouter) in `src/index.tsx`.
+- Added `{/* eslint-disable-next-line camelcase */}` to suppress camelcase lint error on React Router's snake_case API identifiers.
+- Focused test verification: `src/Header.test.tsx` — 21 tests passed, zero React Router future flag warnings.
+- Lint gate: ✓ PASSED.
+- TypeScript gate: ✓ PASSED.
+- Updated `TASK-683-todo.md`: marked item done.

TASK-683-todo.md

@@ -51,6 +51,11 @@ Detailed build-termination findings: `TASK-683-build-investigation.md`.
 - [x] Run A/B proof for build instability (`yarn build` vs `GENERATE_SOURCEMAP=false yarn build`, including `CI=true` variants) and document causal difference
 - [x] Implement production-safe sourcemap disablement fix: add `GENERATE_SOURCEMAP=false` to CI workflow Build step only (not to package.json or Dockerfile to preserve production behavior)
 - [x] Validate CI-style build passes with new GENERATE_SOURCEMAP=false flag (`CI=true GENERATE_SOURCEMAP=false yarn build` -> exit 0, no early-exit marker, 65.73s)
+- [x] Investigate test diagnostic hotspots report (`TASK-683-test-diag-hotspots-2026-03-25.md`) and research all 5 warning classes
+- [x] Fix React Router future flag warnings (26 occurrences): add `future` prop to `BrowserRouter` in `src/index.tsx`
+- [ ] Fix `controlId ignored on FormLabel/FormControl` warnings (306 occurrences): remove redundant `htmlFor`/`id` when `controlId` is on `Form.Group`, or remove `controlId`
+- [ ] Fix `validateDOMNesting` warnings (10 occurrences): fix 4 structural HTML violations across `SignsSearch`, `markup.tsx`, `LineAccumulator`
+- [ ] Fix `act(...)` warnings (173 occurrences): wrap shared test helpers (`changeValue`, `clickNth`) in `act()`, then audit individual test files
 
 ## Notes
 

src/index.tsx

@@ -156,7 +156,8 @@ const root = createRoot(container)
 root.render(
   <ErrorReporterContext.Provider value={errorReporter}>
     <ErrorBoundary>
-      <Router>
+      {/* eslint-disable-next-line camelcase */}
+      <Router future={{ v7_startTransition: true, v7_relativeSplatPath: true }}>
         <div className="mh-100">
           <div>
             <InjectedAuth0Provider>