fix(picker): ARIA combobox — listbox only when results; no tabindex on options

Status/error/empty messages rendered outside <ul role="listbox"> so the
element only contains role="option" children (fixes aria-required-children
axe violation). Options no longer carry tabindex or onkeydown — keyboard
navigation stays on the input per ARIA combobox pattern.

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

frontend/src/lib/document/DocumentPickerDropdown.svelte.spec.ts

@@ -198,3 +198,46 @@ describe('DocumentPickerDropdown — search failure', () => {
 		await expect.element(page.getByText(m.comp_typeahead_error())).toBeInTheDocument();
 	});
 });
+
+describe('DocumentPickerDropdown — ARIA listbox integrity', () => {
+	it('does not render a listbox when results are empty (no aria-required-children violation)', async () => {
+		mockSearchResponse([]);
+		render(DocumentPickerDropdown, { onSelect: vi.fn() });
+
+		await userEvent.fill(page.getByRole('combobox'), 'xyz');
+		await waitForDebounce();
+
+		// no-results message must be visible, but NOT inside a listbox
+		await expect.element(page.getByText(m.comp_typeahead_no_results())).toBeInTheDocument();
+		expect(document.querySelector('[role="listbox"]')).toBeNull();
+	});
+
+	it('does not render a listbox when loading (no aria-required-children violation)', async () => {
+		let resolveSearch!: (v: unknown) => void;
+		vi.stubGlobal(
+			'fetch',
+			vi.fn().mockReturnValue(new Promise((resolve) => (resolveSearch = resolve)))
+		);
+		render(DocumentPickerDropdown, { onSelect: vi.fn() });
+
+		await userEvent.fill(page.getByRole('combobox'), 'Brief');
+
+		// While in-flight, no listbox should exist
+		expect(document.querySelector('[role="listbox"]')).toBeNull();
+		resolveSearch({ ok: true, json: () => Promise.resolve({ items: [] }) });
+	});
+
+	it('option elements do not have tabindex (combobox pattern: focus stays on input)', async () => {
+		mockSearchResponse([docFactory('d1', 'Brief A'), docFactory('d2', 'Brief B')]);
+		render(DocumentPickerDropdown, { onSelect: vi.fn() });
+
+		await userEvent.fill(page.getByRole('combobox'), 'Brief');
+		await waitForDebounce();
+
+		const options = document.querySelectorAll('[role="listbox"] [role="option"]');
+		expect(options.length).toBeGreaterThan(0);
+		options.forEach((opt) => {
+			expect(opt).not.toHaveAttribute('tabindex');
+		});
+	});
+});