fix(compare): stale basket fetch no longer blanks a freshly opened comparison

Opening a compare link while localStorage held a different basket raced a
fetch for the OLD school set against the URL's SSR data; the stale
response replaced comparisonData, so no active school had data and every
section (including the trends chart) vanished until a hard refresh.
Responses from superseded effect runs are now dropped, successful ones
merge instead of replace, and the URL-seed effect re-runs when a
client-side navigation changes ?urns=.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/__tests__/components/ComparisonView.staleFetch.test.tsx

@@ -0,0 +1,107 @@
+/**
+ * Regression: opening a compare link while a DIFFERENT basket is stored must
+ * not blank the page.
+ *
+ * The basket hydrates from localStorage first, which can fire a fetch for the
+ * OLD school set; the URL-seed effect then replaces the basket with the URL's
+ * schools (already covered by SSR data, so no new fetch). When the stale
+ * response for the old set finally lands, it must not clobber the fresh SSR
+ * data — that left every section (including the trends chart) empty until a
+ * hard refresh.
+ */
+
+import { act, render, screen, waitFor } from '@testing-library/react';
+
+import { ComparisonView } from '@/components/ComparisonView';
+import { ComparisonProvider } from '@/context/ComparisonProvider';
+import type { ComparisonData, School } from '@/lib/types';
+
+const fetchComparison = jest.fn();
+jest.mock('@/lib/api', () => ({
+  fetchComparison: (...args: unknown[]) => fetchComparison(...args),
+}));
+jest.mock('@/lib/analytics', () => ({ track: jest.fn() }));
+
+function school(urn: number, name: string): School {
+  return {
+    urn,
+    school_name: name,
+    local_authority: 'Testshire',
+    school_type: 'Community school',
+    rwm_expected_pct: 80,
+    phase: 'Primary',
+  } as School;
+}
+
+function data(urn: number, name: string): ComparisonData {
+  return {
+    school_info: school(urn, name),
+    yearly_data: [{ year: 202425, rwm_expected_pct: 80 }] as ComparisonData['yearly_data'],
+    ofsted: null,
+    census: null,
+    admissions: null,
+    admissions_history: [],
+    deprivation: null,
+  };
+}
+
+// The visitor's previously stored basket (a different school entirely).
+const STORED_SCHOOL = school(900, 'Old Stored School');
+
+// The comparison the URL (and SSR) actually asked for.
+const URL_DATA = {
+  '100': data(100, 'Alpha Primary'),
+  '200': data(200, 'Beta Primary'),
+};
+
+beforeEach(() => {
+  fetchComparison.mockReset();
+  localStorage.clear();
+});
+
+test('a stale fetch for the previously stored basket does not clobber the URL comparison', async () => {
+  localStorage.setItem('selectedSchools', JSON.stringify([STORED_SCHOOL]));
+
+  const pending: Array<(v: unknown) => void> = [];
+  fetchComparison.mockImplementation(() => new Promise((resolve) => pending.push(resolve)));
+
+  render(
+    <ComparisonProvider>
+      <ComparisonView
+        initialData={URL_DATA}
+        initialNationalAverages={{
+          year: 202425,
+          primary: { rwm_expected_pct: 62 },
+          secondary: {},
+          by_year: [],
+        }}
+        initialBenchmarks={undefined}
+        initialUrns={[100, 200]}
+        metrics={[]}
+        selectedMetric="rwm_expected_pct"
+      />
+    </ComparisonProvider>,
+  );
+
+  // The URL's schools render from SSR data once the basket is reseeded.
+  await waitFor(() => {
+    expect(screen.getByRole('heading', { name: 'At a glance' })).toBeInTheDocument();
+  });
+  expect(screen.getAllByText('Alpha Primary').length).toBeGreaterThan(0);
+
+  // The transient stored-basket fetch (for school 900) resolves LATE, after
+  // the basket has moved on to the URL's schools.
+  await act(async () => {
+    for (const resolve of pending) {
+      resolve({
+        comparison: { '900': data(900, 'Old Stored School') },
+        national_averages: { year: 202425, primary: {}, secondary: {}, by_year: [] },
+        benchmarks: undefined,
+      });
+    }
+  });
+
+  // The page must still show the URL comparison — not go blank.
+  expect(screen.getByRole('heading', { name: 'At a glance' })).toBeInTheDocument();
+  expect(screen.getAllByText('Alpha Primary').length).toBeGreaterThan(0);
+});