test(stammbaum): prove the AC8 mobile-centre wiring at the route layer (#703)

Sara/Elicit noted AC8 was proven only as recentreAbove geometry, never as
wired behaviour. Add route-level tests that mock window.matchMedia: a tap
recentres the canvas (mirror effect re-fires) when the mobile breakpoint
matches, and leaves the view untouched on desktop where the side panel is a
flex sibling that never overlaps the canvas.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/stammbaum/page.svelte.test.ts

@@ -24,7 +24,28 @@ vi.mock('$app/navigation', () => ({
 	goto: vi.fn()
 }));
 
-afterEach(cleanup);
+// The page reads window.matchMedia('(max-width: 767px)').matches at init to
+// decide whether to centre a tapped person above the bottom sheet (#703 AC8).
+// Make the mock query-aware so only the mobile breakpoint flips; other media
+// queries (e.g. prefers-reduced-motion) keep their benign default.
+const originalMatchMedia = window.matchMedia;
+function mockMatchMedia(isMobile: boolean) {
+	window.matchMedia = ((query: string) => ({
+		matches: query.includes('max-width') ? isMobile : false,
+		media: query,
+		onchange: null,
+		addEventListener: () => {},
+		removeEventListener: () => {},
+		addListener: () => {},
+		removeListener: () => {},
+		dispatchEvent: () => false
+	})) as unknown as typeof window.matchMedia;
+}
+
+afterEach(() => {
+	cleanup();
+	window.matchMedia = originalMatchMedia;
+});
 
 async function loadComponent() {
 	return (await import('./+page.svelte')).default;
@@ -35,6 +56,12 @@ const sampleNodes = [
 	{ id: 'p-2', firstName: 'Bert', lastName: 'Schmidt', displayName: 'Bert Schmidt' }
 ];
 
+// Typed family nodes for the AC8 tests — familyMember is required on the DTO.
+const familyNodes = [
+	{ id: 'p-1', displayName: 'Anna Schmidt', familyMember: true },
+	{ id: 'p-2', displayName: 'Bert Schmidt', familyMember: true }
+];
+
 describe('stammbaum page', () => {
 	it('shows the empty state when there are no family nodes', async () => {
 		mockPage.url = new URL('http://localhost/stammbaum');
@@ -125,4 +152,44 @@ describe('stammbaum page', () => {
 		expect(url.searchParams.has('cx')).toBe(true);
 		expect(url.searchParams.has('cy')).toBe(true);
 	});
+
+	// AC8 — the tapped person must clear the bottom sheet on a phone, but the
+	// desktop side panel is a flex sibling that never overlaps the canvas, so no
+	// centring should fire there. These tests prove the matchMedia gate around
+	// selectPerson, not just the recentreAbove geometry (covered in panZoom.test).
+	it('recentres the tapped person when matchMedia reports mobile (#703 AC8)', async () => {
+		mockMatchMedia(true);
+		mockPage.url = new URL('http://localhost/stammbaum');
+		const Stammbaum = await loadComponent();
+		render(Stammbaum, {
+			props: { data: { nodes: familyNodes, edges: [], initialView: DEFAULT_VIEW } }
+		});
+		// Let the mount-time URL mirror settle, then isolate the tap's effect.
+		await vi.waitFor(() => expect(replaceState).toHaveBeenCalled());
+		replaceState.mockClear();
+
+		await page.getByRole('button', { name: 'Anna Schmidt' }).click();
+
+		// The mobile tap recentres the canvas → the view changes → the ?cx&cy&z
+		// mirror effect re-fires. (Desktop, below, leaves the view untouched.)
+		await vi.waitFor(() => expect(replaceState).toHaveBeenCalled());
+	});
+
+	it('does not recentre on tap when matchMedia reports desktop (#703 AC8)', async () => {
+		mockMatchMedia(false);
+		mockPage.url = new URL('http://localhost/stammbaum');
+		const Stammbaum = await loadComponent();
+		render(Stammbaum, {
+			props: { data: { nodes: familyNodes, edges: [], initialView: DEFAULT_VIEW } }
+		});
+		await vi.waitFor(() => expect(replaceState).toHaveBeenCalled());
+		replaceState.mockClear();
+
+		await page.getByRole('button', { name: 'Anna Schmidt' }).click();
+
+		// The tap registers — the desktop side panel opens — but no recentre fires,
+		// so the view never changes and the mirror effect stays silent.
+		await expect.element(page.getByRole('complementary')).toBeVisible();
+		expect(replaceState).not.toHaveBeenCalled();
+	});
 });