#424

frontend/src/lib/help.test.ts

@@ -60,4 +60,29 @@ describe ('Help file catalogue', () => {
     expect (await api.getHelpEdition ('guide', 'ja', '2026-10-02.11')).toBeNull ()
     expect (await api.getHelpEditionSource ('guide', 'ja', '2026-10-02.11')).toBeNull ()
   })
+
+  it ('compares raw H1 and body lines in the requested direction', async () => {
+    const { api } = createHelpFixture ()
+    const forward = await api.getHelpDiff ('guide', 'ja', '2026-10-01', '2026-10-02.10')
+    expect (forward?.from.title).toBe ('旧ホーム')
+    expect (forward?.to.title).toBe ('ホーム第十版')
+    expect (forward?.lines).toEqual (expect.arrayContaining ([
+	{ type: 'removed', content: '# 旧ホーム\n' },
+	{ type: 'added', content: '# ホーム第十版\n' },
+	{ type: 'unchanged', content: '\n' }]))
+    const reverse = await api.getHelpDiff ('guide', 'ja', '2026-10-02.10', '2026-10-01')
+    expect (reverse?.lines).toEqual (expect.arrayContaining ([
+	{ type: 'added', content: '# 旧ホーム\n' },
+	{ type: 'removed', content: '# ホーム第十版\n' }]))
+  })
+
+  it.each ([['missing', 'ja', '2026-10-01', '2026-10-02.10'],
+      ['guide', 'ja', '2026-10-01', '2026-10-02.11'],
+      ['guide', 'en', '2026-10-01', '2026-10-02']]) (
+      'rejects invalid diff editions for %s/%s without reading sources',
+      async (id, language, from, to) => {
+	const { api, raw } = createHelpFixture ()
+	expect (await api.getHelpDiff (id, language, from, to)).toBeNull ()
+	Object.values (raw).forEach (load => expect (load).not.toHaveBeenCalled ())
+      })
 })

frontend/src/lib/helpPrefetch.test.tsx

@@ -6,6 +6,7 @@ import { beforeEach, describe, expect, it, vi } from 'vitest'
 import PrefetchLink from '@/components/PrefetchLink'
 import * as prefetchers from '@/lib/prefetchers'
 import HelpDetailPage from '@/pages/help/HelpDetailPage'
+import HelpDiffPage from '@/pages/help/HelpDiffPage'
 import { helpFixture } from '@/test/helpFixtures'
 import { renderWithProviders } from '@/test/render'
 
@@ -76,6 +77,29 @@ describe ('Help prefetch integration', () => {
     Object.values (helpFixture.modules).forEach (load => expect (load).not.toHaveBeenCalled ())
   })
 
+  it ('reuses two prefetched raw editions when displaying their diff', async () => {
+    const path = '/help/guide/diff?from=2026-10-01&to=2026-10-02.10'
+    const { queryClient } = renderWithProviders (
+	<>
+	  <PrefetchLink to={path}>diff</PrefetchLink>
+	  <Routes>
+	    <Route path="/" element={null}/>
+	    <Route path="/help/:identifier/diff" element={<HelpDiffPage/>}/>
+	  </Routes>
+	</>)
+    await Promise.all ([prefetchers.prefetchForURL (queryClient, path),
+	prefetchers.prefetchForURL (queryClient, path)])
+    fireEvent.click (screen.getByRole ('link', { name: 'diff' }))
+    expect (await screen.findByRole ('link', { name: '旧ホーム (2026-10-01)' }))
+	.toHaveAttribute ('href', '/help/guide?edition=2026-10-01')
+    expect (screen.getByText ('+ # ホーム第十版')).toBeInTheDocument ()
+    Object.entries (helpFixture.raw).forEach (([name, load]) => {
+      const expected = /\/ja\/(2026-10-01|2026-10-02\.10)\.mdx$/.test (name) ? 1 : 0
+      expect (load).toHaveBeenCalledTimes (expected)
+    })
+    Object.values (helpFixture.modules).forEach (load => expect (load).not.toHaveBeenCalled ())
+  })
+
   it.each (['intent', 'off']) ('respects %s intent settings and still prefetches clicks',
       async mode => {
 	settings.linkPreload = mode

frontend/src/test/helpFixtures.tsx

@@ -4,7 +4,7 @@ import { vi } from 'vitest'
 
 import source from '@/lib/help.ts?raw'
 
-import type { ReactElement } from 'react'
+import type { ComponentType } from 'react'
 
 import type * as Help from '@/lib/help'
 
@@ -38,11 +38,11 @@ export const createHelpFixture = () => {
       vi.fn (async () => `# ${title}\n\n${body}`)]))
   const moduleLoads = Object.fromEntries (published.map (([id, language, edition, title, body]) => [
       `../../help/${id}/editions/${language}/${edition}.mdx`,
-      vi.fn (async (): Promise<{ default: () => ReactElement }> => ({
+      vi.fn (async (): Promise<{ default: ComponentType }> => ({
 	  default: () => createElement ('div', null,
 	      createElement ('h1', null, title), createElement ('p', null, body)) }))]))
   // Dynamic imports reuse a module namespace even when called more than once.
-  const caches: Record<string, { promise?: Promise<{ default: () => ReactElement }> }> = { }
+  const caches: Record<string, { promise?: Promise<{ default: ComponentType }> }> = { }
   const modules = Object.fromEntries (Object.entries (moduleLoads).map (([path, load]) => {
     caches[path] = { }
     return [path, vi.fn (() => caches[path].promise ??= load ())]