diff --git a/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts b/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
index 5a97fb18..7ead48f9 100644
--- a/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
+++ b/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
@@ -1,6 +1,7 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
+import { flushSync, mount, unmount } from 'svelte';
 import MentionDropdown from './MentionDropdown.svelte';
 import MentionDropdownFixture from './MentionDropdown.test-fixture.svelte';
 import { m } from '$lib/paraglide/messages.js';
@@ -218,3 +219,121 @@ describe('MentionDropdown — search input', () => {
 		await expect.element(page.getByRole('searchbox')).toHaveValue('Walter');
 	});
 });
+
+// ─── ArrowDown via exported onKeyDown (Sara #3 on PR #629) ──────────────────
+//
+// In production, Tiptap intercepts ArrowDown/ArrowUp/Enter at the editor level
+// and forwards them to the dropdown via its exported onKeyDown(event) function
+// — the dropdown itself has no DOM keydown listener. This test exercises the
+// same export so a regression in highlightedIndex/selection logic is caught
+// at the unit level. The full E2E focus-chain test is deferred to a separate
+// issue (Playwright).
+
+describe('MentionDropdown — onKeyDown forwarding', () => {
+	it('ArrowDown advances aria-selected to the next option in the listbox', async () => {
+		const container = document.createElement('div');
+		document.body.appendChild(container);
+		const instance = mount(MentionDropdown, {
+			target: container,
+			props: {
+				model: baseModel({
+					items: [makePerson('p1', 'Anna Schmidt'), makePerson('p2', 'Bert Meier')]
+				})
+			}
+		});
+		try {
+			const exports = instance as unknown as { onKeyDown: (e: KeyboardEvent) => boolean };
+
+			// First option starts highlighted.
+			const first = container.querySelector('[data-test-person-id="p1"]') as HTMLElement;
+			const second = container.querySelector('[data-test-person-id="p2"]') as HTMLElement;
+			expect(first.getAttribute('aria-selected')).toBe('true');
+			expect(second.getAttribute('aria-selected')).toBe('false');
+
+			let consumed = false;
+			flushSync(() => {
+				consumed = exports.onKeyDown(new KeyboardEvent('keydown', { key: 'ArrowDown' }));
+			});
+			expect(consumed).toBe(true);
+
+			expect(first.getAttribute('aria-selected')).toBe('false');
+			expect(second.getAttribute('aria-selected')).toBe('true');
+		} finally {
+			unmount(instance);
+			container.remove();
+		}
+	});
+
+	it('ArrowUp wraps from the first option to the last', async () => {
+		const container = document.createElement('div');
+		document.body.appendChild(container);
+		const instance = mount(MentionDropdown, {
+			target: container,
+			props: {
+				model: baseModel({
+					items: [makePerson('p1', 'Anna Schmidt'), makePerson('p2', 'Bert Meier')]
+				})
+			}
+		});
+		try {
+			const exports = instance as unknown as { onKeyDown: (e: KeyboardEvent) => boolean };
+
+			let consumed = false;
+			flushSync(() => {
+				consumed = exports.onKeyDown(new KeyboardEvent('keydown', { key: 'ArrowUp' }));
+			});
+			expect(consumed).toBe(true);
+
+			const second = container.querySelector('[data-test-person-id="p2"]') as HTMLElement;
+			expect(second.getAttribute('aria-selected')).toBe('true');
+		} finally {
+			unmount(instance);
+			container.remove();
+		}
+	});
+
+	it('Enter invokes model.command with the currently highlighted item', async () => {
+		const command = vi.fn();
+		const container = document.createElement('div');
+		document.body.appendChild(container);
+		const instance = mount(MentionDropdown, {
+			target: container,
+			props: {
+				model: baseModel({
+					items: [makePerson('p1', 'Anna Schmidt'), makePerson('p2', 'Bert Meier')],
+					command
+				})
+			}
+		});
+		try {
+			const exports = instance as unknown as { onKeyDown: (e: KeyboardEvent) => boolean };
+
+			const consumed = exports.onKeyDown(new KeyboardEvent('keydown', { key: 'Enter' }));
+			expect(consumed).toBe(true);
+			expect(command).toHaveBeenCalledTimes(1);
+			expect(command.mock.calls[0][0].id).toBe('p1');
+		} finally {
+			unmount(instance);
+			container.remove();
+		}
+	});
+
+	it('Escape returns false so the suggestion plugin can handle it', async () => {
+		const container = document.createElement('div');
+		document.body.appendChild(container);
+		const instance = mount(MentionDropdown, {
+			target: container,
+			props: {
+				model: baseModel({ items: [makePerson('p1', 'Anna Schmidt')] })
+			}
+		});
+		try {
+			const exports = instance as unknown as { onKeyDown: (e: KeyboardEvent) => boolean };
+			const consumed = exports.onKeyDown(new KeyboardEvent('keydown', { key: 'Escape' }));
+			expect(consumed).toBe(false);
+		} finally {
+			unmount(instance);
+			container.remove();
+		}
+	});
+});