fix(stammbaum): order keyboard tab stops by visual layout, not DB order (#718)

Person nodes rendered in `nodes` array order (backend/DB row order), so
Tab focus hopped between nodes unrelated to their on-screen position,
failing WCAG 2.4.3 Focus Order (Level A).

Render the node loop in reading order instead: sort by layout y (top
generation first) then x (left-to-right within a row), via a
`nodesInReadingOrder` derived. Nodes without a layout position sort last
(mirroring the `{#if pos}` guard); node.id is the final tie-break for a
total, deterministic comparator. Shift+Tab and reload-stability fall out
for free (reversed render order; x/y independent of backend order).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumTree.svelte

@@ -70,6 +70,31 @@ let {
 
 const layout = $derived.by<Layout>(() => buildLayout(nodes, edges));
 
+// Keyboard tab order follows DOM source order (#718). Render the nodes in
+// reading order — top generation first (by layout y), left-to-right within a
+// row (by layout x) — so focus sweeps the visual grid instead of the backend
+// row order in which `nodes` arrives. Nodes without a layout position sort last
+// (mirroring the `{#if pos}` render guard); node.id is the final tie-break for a
+// total, deterministic comparator.
+//
+// LOAD-BEARING: relies on every node in a generation sharing the same y
+// (buildLayout assigns y = g * (NODE_H + ROW_GAP)). If the layout is ever
+// changed to stagger nodes vertically within a generation, this y-then-x sort
+// would silently regress tab order — revisit here.
+const nodesInReadingOrder = $derived.by(() =>
+	[...nodes].sort((a, b) => {
+		const pa = layout.positions.get(a.id);
+		const pb = layout.positions.get(b.id);
+		const ay = pa ? pa.y : Infinity;
+		const by = pb ? pb.y : Infinity;
+		if (ay !== by) return ay - by;
+		const ax = pa ? pa.x : Infinity;
+		const bx = pb ? pb.x : Infinity;
+		if (ax !== bx) return ax - bx;
+		return a.id.localeCompare(b.id);
+	})
+);
+
 // Lineage highlight (#703). The adjacency index is rebuilt only when the edges
 // change; the cheap walk re-runs whenever the selection changes. A null
 // highlight (no selection) means full strength everywhere — nothing dims.
@@ -312,8 +337,8 @@ function handleCanvasKey(event: KeyboardEvent) {
 			isConnectorActive={isConnectorActive}
 		/>
 
-		<!-- Nodes -->
+		<!-- Nodes (reading order so keyboard Tab follows the visual grid, #718) -->
-		{#each nodes as node (node.id)}
+		{#each nodesInReadingOrder as node (node.id)}
 			{@const pos = layout.positions.get(node.id)}
 			{#if pos}
 				<StammbaumNode

frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts

@@ -1086,3 +1086,123 @@ describe('StammbaumTree lineage highlight (#703)', () => {
 		expect(dimmedConnectorCount()).toBe(1);
 	});
 });
+
+describe('StammbaumTree keyboard tab order (#718)', () => {
+	// Tab order follows DOM source order, so the node <g role="button"> elements
+	// must render in reading order — top generation first, left-to-right within a
+	// row — regardless of the order nodes arrive in the `nodes` array (which is
+	// backend/DB order). Scope to in-SVG role="button" elements: the canvas and
+	// the controls/side panel are also focusable, so a whole-page Tab count would
+	// be brittle (see the issue's testing note).
+	const WALTER = '00000000-0000-0000-0000-0000000000a1';
+	const EUGENIE = '00000000-0000-0000-0000-0000000000a2';
+	const CLARA = '00000000-0000-0000-0000-0000000000a3';
+	const HANS = '00000000-0000-0000-0000-0000000000a4';
+
+	// Walter ↔ Eugenie (gen 0); their children Clara + Hans (gen 1). buildLayout
+	// sorts each generation alphabetically, so the deterministic visual order is
+	// Eugenie, Walter (top row) then Clara, Hans (next row).
+	const FAMILY_EDGES = [
+		{
+			id: 'sp',
+			personId: WALTER,
+			relatedPersonId: EUGENIE,
+			personDisplayName: 'Walter',
+			relatedPersonDisplayName: 'Eugenie',
+			relationType: 'SPOUSE_OF'
+		},
+		{
+			id: 'p1',
+			personId: WALTER,
+			relatedPersonId: CLARA,
+			personDisplayName: 'Walter',
+			relatedPersonDisplayName: 'Clara',
+			relationType: 'PARENT_OF'
+		},
+		{
+			id: 'p2',
+			personId: EUGENIE,
+			relatedPersonId: CLARA,
+			personDisplayName: 'Eugenie',
+			relatedPersonDisplayName: 'Clara',
+			relationType: 'PARENT_OF'
+		},
+		{
+			id: 'p3',
+			personId: WALTER,
+			relatedPersonId: HANS,
+			personDisplayName: 'Walter',
+			relatedPersonDisplayName: 'Hans',
+			relationType: 'PARENT_OF'
+		},
+		{
+			id: 'p4',
+			personId: EUGENIE,
+			relatedPersonId: HANS,
+			personDisplayName: 'Eugenie',
+			relatedPersonDisplayName: 'Hans',
+			relationType: 'PARENT_OF'
+		}
+	];
+
+	function nodeLabelsInDomOrder(): string[] {
+		const svg = document.querySelector('svg')!;
+		return Array.from(svg.querySelectorAll('g[role="button"]')).map(
+			(g) => (g.getAttribute('aria-label') ?? '').split(',')[0]
+		);
+	}
+
+	it('renders node tab stops in reading order, not backend array order', () => {
+		// Deliberately scrambled (not reading order): Hans (gen 1), Eugenie
+		// (gen 0), Clara (gen 1), Walter (gen 0). Source order alone would tab
+		// gen1 → gen0 → gen1 → gen0.
+		render(StammbaumTree, {
+			nodes: [
+				{ id: HANS, displayName: 'Hans', familyMember: true },
+				{ id: EUGENIE, displayName: 'Eugenie', familyMember: true },
+				{ id: CLARA, displayName: 'Clara', familyMember: true },
+				{ id: WALTER, displayName: 'Walter', familyMember: true }
+			],
+			edges: FAMILY_EDGES,
+			selectedId: null,
+			panZoom: { x: 0, y: 0, z: 1 },
+			showGutter: false,
+			onSelect: () => {}
+		});
+
+		// Top generation left-to-right, then next generation left-to-right.
+		expect(nodeLabelsInDomOrder()).toEqual(['Eugenie', 'Walter', 'Clara', 'Hans']);
+	});
+
+	it('orders tab stops by rendered position regardless of input order', () => {
+		// Self-validating against the actual layout: DOM order of the node groups
+		// must equal the same groups sorted by (y, then x). Independent of the
+		// alphabetical assumption above, so it survives layout-internal changes.
+		render(StammbaumTree, {
+			nodes: [
+				{ id: CLARA, displayName: 'Clara', familyMember: true },
+				{ id: WALTER, displayName: 'Walter', familyMember: true },
+				{ id: HANS, displayName: 'Hans', familyMember: true },
+				{ id: EUGENIE, displayName: 'Eugenie', familyMember: true }
+			],
+			edges: FAMILY_EDGES,
+			selectedId: null,
+			panZoom: { x: 0, y: 0, z: 1 },
+			showGutter: false,
+			onSelect: () => {}
+		});
+
+		const svg = document.querySelector('svg')!;
+		const placed = Array.from(svg.querySelectorAll('g[role="button"]')).map((g) => {
+			const t = g.getAttribute('transform') ?? '';
+			const m = t.match(/translate\((-?[\d.]+)[,\s]+(-?[\d.]+)\)/);
+			return {
+				label: (g.getAttribute('aria-label') ?? '').split(',')[0],
+				x: m ? parseFloat(m[1]) : Infinity,
+				y: m ? parseFloat(m[2]) : Infinity
+			};
+		});
+		const readingOrder = [...placed].sort((a, b) => a.y - b.y || a.x - b.x);
+		expect(placed.map((n) => n.label)).toEqual(readingOrder.map((n) => n.label));
+	});
+});