diff --git a/frontend/src/lib/person/genealogy/layout/tidyTree.test.ts b/frontend/src/lib/person/genealogy/layout/tidyTree.test.ts
index 18c05c3f..b68d63a4 100644
--- a/frontend/src/lib/person/genealogy/layout/tidyTree.test.ts
+++ b/frontend/src/lib/person/genealogy/layout/tidyTree.test.ts
@@ -10,6 +10,14 @@ function leaf(id: string, width = W): TidyNode {
 	return { id, width, children: [] };
 }
 
+function node(id: string, children: TidyNode[], width = W): TidyNode {
+	return { id, width, children };
+}
+
+function center(x: Map<string, number>, n: TidyNode): number {
+	return x.get(n.id)! + n.width / 2;
+}
+
 describe('tidyTree — leaf base case', () => {
 	it('a single leaf lays out at x = 0', () => {
 		const a = leaf('a');
@@ -17,3 +25,23 @@ describe('tidyTree — leaf base case', () => {
 		expect(x.get('a')).toBe(0);
 	});
 });
+
+describe('tidyTree — ancestor centring', () => {
+	it('a parent is centred over the span of its two children', () => {
+		const c1 = leaf('c1');
+		const c2 = leaf('c2');
+		const p = node('p', [c1, c2]);
+		const x = layoutForest([p], GAP);
+
+		const pc = center(x, p);
+		const lo = center(x, c1);
+		const hi = center(x, c2);
+		// Parent sits exactly at the midpoint of its children's centres …
+		expect(pc).toBe((lo + hi) / 2);
+		// … which is within their span (the named-bug guard generalises this).
+		expect(pc).toBeGreaterThanOrEqual(Math.min(lo, hi));
+		expect(pc).toBeLessThanOrEqual(Math.max(lo, hi));
+		// Children do not overlap.
+		expect(Math.abs(x.get('c2')! - x.get('c1')!)).toBeGreaterThanOrEqual(W + GAP);
+	});
+});