diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
index a3596e3b..d474b768 100644
--- a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
+++ b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
@@ -1047,3 +1047,133 @@ describe('PersonMentionEditor — #628 re-edit pencil', () => {
 		await expect.element(dismissButton()).toBeVisible();
 	});
 });
+
+// ─── #628 AC-6: at most one mention dropdown open at a time ───────────────────
+
+describe('PersonMentionEditor — #628 AC-6 single-dropdown invariant', () => {
+	const TWO_MENTIONS = [
+		{ personId: 'p-aug', displayName: 'Aug' },
+		{ personId: 'p-bert', displayName: 'Bert' }
+	];
+
+	function mentionButtons(): HTMLButtonElement[] {
+		return Array.from(
+			document.querySelectorAll('[data-type="mention"] button')
+		) as HTMLButtonElement[];
+	}
+
+	function click(el: HTMLElement) {
+		el.dispatchEvent(new MouseEvent('click', { bubbles: true, cancelable: true }));
+	}
+
+	function listboxCount() {
+		return document.querySelectorAll('[role="listbox"]').length;
+	}
+
+	it('pencil → pencil: opening a second mention pencil closes the first', async () => {
+		mockFetchEmpty();
+		renderHost({ value: '@Aug @Bert ', mentionedPersons: TWO_MENTIONS });
+		await vi.waitFor(() => expect(mentionButtons().length).toBe(2));
+		const [pencilA, pencilB] = mentionButtons();
+
+		click(pencilA);
+		await vi.waitFor(() => expect(listboxCount()).toBe(1));
+
+		click(pencilB);
+		await vi.waitFor(async () => {
+			expect(listboxCount()).toBe(1);
+			await expect.element(page.getByRole('searchbox')).toHaveValue('Bert');
+		});
+	});
+
+	it('fresh-@ → pencil: activating a pencil closes the open fresh-@ dropdown', async () => {
+		mockFetchEmpty();
+		renderHost({ value: '@Aug ', mentionedPersons: [{ personId: 'p-aug', displayName: 'Aug' }] });
+		await vi.waitFor(() => expect(mentionButtons().length).toBe(1));
+
+		// Open a fresh-@ dropdown (prefilled with the typed query "x").
+		await userEvent.type(page.getByRole('textbox'), '@x');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toHaveValue('x');
+		});
+
+		// Now activate the saved mention's pencil — the fresh dropdown must give way.
+		click(mentionButtons()[0]);
+		await vi.waitFor(async () => {
+			expect(listboxCount()).toBe(1);
+			await expect.element(page.getByRole('searchbox')).toHaveValue('Aug');
+		});
+	});
+
+	it('pencil → fresh-@: typing @ in the editor closes the open re-edit dropdown', async () => {
+		mockFetchEmpty();
+		renderHost({ value: '@Aug ', mentionedPersons: [{ personId: 'p-aug', displayName: 'Aug' }] });
+		await vi.waitFor(() => expect(mentionButtons().length).toBe(1));
+
+		click(mentionButtons()[0]);
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toHaveValue('Aug');
+		});
+
+		// Typing @ in the editor starts a fresh suggestion — the re-edit dropdown
+		// must give way to it (single owner, both orderings).
+		await userEvent.type(page.getByRole('textbox'), '@y');
+		await vi.waitFor(async () => {
+			expect(listboxCount()).toBe(1);
+			await expect.element(page.getByRole('searchbox')).toHaveValue('y');
+		});
+	});
+
+	it('regression: fresh-@ still inserts the typed text as displayName (#380 AC-1 intact)', async () => {
+		mockFetchWithPersons();
+		const host = renderHost();
+		await userEvent.type(page.getByRole('textbox'), '@Aug');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('option', { name: /Auguste Raddatz/ })).toBeVisible();
+		});
+		const option = (await page
+			.getByRole('option', { name: /Auguste Raddatz/ })
+			.element()) as HTMLElement;
+		option.dispatchEvent(new MouseEvent('mousedown', { bubbles: true, cancelable: true }));
+
+		await vi.waitFor(() => {
+			expect(host.snapshot.mentionedPersons).toEqual([{ personId: 'p-aug', displayName: 'Aug' }]);
+		});
+	});
+
+	it('discards a stale fetch from a superseded open (open A → open B → A resolves)', async () => {
+		// A's fetch hangs; B's resolves empty. When A finally resolves with a
+		// person, the request-token guard must discard it so B's dropdown is not
+		// repopulated. Deterministic — no sleeps.
+		let resolveA!: (v: { ok: boolean; json: () => Promise<{ items: Person[] }> }) => void;
+		const pendingA = new Promise<{ ok: boolean; json: () => Promise<{ items: Person[] }> }>((r) => {
+			resolveA = r;
+		});
+		let call = 0;
+		const fetchMock = vi.fn().mockImplementation(() => {
+			call += 1;
+			if (call === 1) return pendingA;
+			return Promise.resolve({ ok: true, json: () => Promise.resolve({ items: [] }) });
+		});
+		vi.stubGlobal('fetch', fetchMock);
+
+		renderHost({ value: '@Aug @Bert ', mentionedPersons: TWO_MENTIONS });
+		await vi.waitFor(() => expect(mentionButtons().length).toBe(2));
+		const [pencilA, pencilB] = mentionButtons();
+
+		click(pencilA);
+		await vi.waitFor(() => {
+			expect(fetchMock).toHaveBeenCalledWith(expect.stringContaining('q=Aug'));
+		});
+
+		click(pencilB);
+		await vi.waitFor(() => {
+			expect(fetchMock).toHaveBeenCalledWith(expect.stringContaining('q=Bert'));
+		});
+
+		// A's stale response arrives last — it must NOT repopulate B's dropdown.
+		resolveA({ ok: true, json: () => Promise.resolve({ items: [AUGUSTE] }) });
+		await tick();
+		await expect.element(page.getByText('Auguste Raddatz')).not.toBeInTheDocument();
+	});
+});