feat(suggest): wire autosuggest into the search box behind a flag

Off means off — no combobox role, no listener, no fetch. A test asserts
the absence of the request, not just the absence of the dropdown,
because a hidden-but-fetching control would still be spending the rate
limit on a feature nobody can see.

Enter with no active option falls through to the form's submit handler
and searches the typed text exactly as before. The existing behaviour is
preserved, not replaced, and that has its own test.

Suppressed once the value parses as a postcode: the box takes a name OR
a postcode, and suggesting schools during postcode entry fights the user.

.omniBoxContainer gains position: relative — the dropdown is absolutely
positioned and without it would have anchored to the page instead.

Four render sites, all wired: page.tsx renders HomeView in the success
path AND the catch fallback, and HomeView renders FilterBar as hero AND
sticky. Missing any one would make the flag silently do nothing
somewhere.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

nextjs-app/__tests__/components/FilterBarSuggest.test.tsx

@@ -0,0 +1,76 @@
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { FilterBar } from '@/components/FilterBar';
+
+const push = jest.fn();
+jest.mock('next/navigation', () => ({
+  useRouter: () => ({ push, replace: jest.fn(), prefetch: jest.fn() }),
+  usePathname: () => '/',
+  useSearchParams: () => new URLSearchParams(),
+}));
+
+const FILTERS = {
+  local_authorities: [], school_types: [], years: [], phases: [],
+  genders: [], admissions_policies: [],
+};
+
+const realFetch = global.fetch;
+beforeEach(() => {
+  global.fetch = jest.fn(async () => ({
+    ok: true,
+    json: async () => ({ suggestions: [{
+      urn: 100010, school_name: 'Brecknock Primary School',
+      local_authority: 'Camden', postcode: 'NW1 1AA',
+      phase: 'Primary', school_type: 'Community school' }] }),
+  })) as unknown as typeof fetch;
+  push.mockClear();
+});
+afterEach(() => { global.fetch = realFetch; });
+
+describe('FilterBar autosuggest', () => {
+  it('is a combobox only when the flag is on', () => {
+    const { rerender } = render(<FilterBar filters={FILTERS} autosuggest={false} />);
+    expect(screen.queryByRole('combobox')).not.toBeInTheDocument();
+    rerender(<FilterBar filters={FILTERS} autosuggest />);
+    expect(screen.getByRole('combobox')).toBeInTheDocument();
+  });
+
+  it('makes no request while the flag is off', async () => {
+    // Off means off: no listener, no fetch, no markup.
+    render(<FilterBar filters={FILTERS} autosuggest={false} />);
+    await userEvent.type(screen.getByPlaceholderText(/School name or postcode/i),
+      'brecknock');
+    expect(global.fetch).not.toHaveBeenCalled();
+  });
+
+  it('shows suggestions and navigates when one is chosen', async () => {
+    render(<FilterBar filters={FILTERS} autosuggest />);
+    await userEvent.type(screen.getByRole('combobox'), 'brecknock');
+    const option = await screen.findByRole('option', { name: /Brecknock/ });
+    await userEvent.click(option);
+    expect(push).toHaveBeenCalledWith(
+      expect.stringContaining('/school/100010'));
+  });
+
+  it('suppresses suggestions once the value is a postcode', async () => {
+    // The box takes a name OR a postcode; suggestions must get out of the way.
+    //
+    // fireEvent.change, not userEvent.type: typing sets "N", "NW", "NW1"... and
+    // "NW1" is not a postcode, so a request for it is correct behaviour. Only
+    // the settled value is the assertion, so set it in one go.
+    render(<FilterBar filters={FILTERS} autosuggest />);
+    fireEvent.change(screen.getByRole('combobox'), { target: { value: 'NW1 1AA' } });
+    await new Promise((r) => setTimeout(r, 300));   // past the 200ms debounce
+    expect(global.fetch).not.toHaveBeenCalled();
+  });
+
+  it('Enter with no active option still submits the free-text search', async () => {
+    // The existing behaviour is preserved, not replaced.
+    render(<FilterBar filters={FILTERS} autosuggest />);
+    const input = screen.getByRole('combobox');
+    await userEvent.type(input, 'brecknock{Enter}');
+    // updateURL pushes inside startTransition, so the call is not synchronous.
+    await waitFor(() => expect(push).toHaveBeenCalledWith(
+      expect.stringContaining('search=brecknock')));
+  });
+});