Fix stale contact suggestions reopening

hide() did not invalidate in-flight search requests. Clearing
the input or picking a contact while a request was pending let
the old response re-render and pop the suggestion list back
open. Bump requestSeq in hide() so every dismissal retires
pending responses.

Add a Vitest + jsdom harness with black-box tests for the
invitee widget, covering the stale-response race both on
field clear and on contact pick.

tests/js/invitees.test.mjs

@@ -0,0 +1,128 @@
+import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from 'vitest';
+
+const DEBOUNCE_MS = 250;
+const SEARCH_URL = '/apps/atocal/contact/search';
+const BOB = { name: 'Bob', email: 'bob@example.com' };
+const ZOE = { name: 'Zoe', email: 'zoe@example.com' };
+
+let input;
+let list;
+
+function response(contacts) {
+  return Promise.resolve({
+    ok: true,
+    json: () => Promise.resolve(contacts),
+  });
+}
+
+/* Promise we can settle manually, to emulate an in-flight request */
+function pendingResponse() {
+  let resolve;
+  const promise = new Promise((res) => {
+    resolve = res;
+  });
+
+  return { promise, resolve };
+}
+
+function type(fragment) {
+  input.value = fragment;
+  input.dispatchEvent(new Event('input', { bubbles: true }));
+}
+
+/* Let the debounce fire and the promise chain settle */
+async function settle(ms = DEBOUNCE_MS) {
+  await vi.advanceTimersByTimeAsync(ms);
+}
+
+beforeAll(async () => {
+  document.body.innerHTML =
+    '<span><input id="atocal-attendees" data-search-url="' + SEARCH_URL + '"></span>';
+
+  await import('../../js/invitees.js');
+
+  input = document.getElementById('atocal-attendees');
+  list = document.querySelector('.atocal-suggestions');
+});
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  vi.stubGlobal('fetch', vi.fn());
+});
+
+afterEach(async () => {
+  /* Empty fragment hides the list and cancels pending debounces */
+  type('');
+  await settle(0);
+  vi.useRealTimers();
+  vi.unstubAllGlobals();
+});
+
+describe('invitee suggestions', () => {
+  it('renders matching contacts', async () => {
+    fetch.mockReturnValue(response([BOB]));
+
+    type('bo');
+    await settle();
+
+    expect(fetch).toHaveBeenCalledTimes(1);
+    expect(list.hidden).toBe(false);
+    expect(list.querySelectorAll('[role="option"]')).toHaveLength(1);
+    expect(list.textContent).toContain('bob@example.com');
+    expect(input.getAttribute('aria-expanded')).toBe('true');
+  });
+
+  it('ignores responses that arrive after the field was cleared', async () => {
+    const inFlight = pendingResponse();
+    fetch.mockReturnValue(inFlight.promise);
+
+    type('bo');
+    await settle();
+
+    /* User clears the field while the request is still in flight */
+    type('');
+
+    inFlight.resolve({
+      ok: true,
+      json: () => Promise.resolve([BOB]),
+    });
+    await settle(0);
+
+    expect(list.hidden).toBe(true);
+    expect(list.children).toHaveLength(0);
+    expect(input.getAttribute('aria-expanded')).toBe('false');
+  });
+
+  it('ignores responses that arrive after a contact was picked', async () => {
+    const second = pendingResponse();
+    fetch
+      .mockReturnValueOnce(response([BOB]))
+      .mockReturnValueOnce(second.promise);
+
+    type('bo');
+    await settle();
+    expect(list.hidden).toBe(false);
+
+    /* Typing more triggers a second request */
+    type('bob');
+    await settle();
+
+    /* User picks the visible suggestion while the request is in flight */
+    list.querySelector('button').dispatchEvent(
+      new MouseEvent('mousedown', { bubbles: true, cancelable: true })
+    );
+
+    expect(input.value).toBe('bob@example.com, ');
+    expect(list.hidden).toBe(true);
+
+    /* The stale response must not reopen the list */
+    second.resolve({
+      ok: true,
+      json: () => Promise.resolve([ZOE]),
+    });
+    await settle(0);
+
+    expect(list.hidden).toBe(true);
+    expect(list.children).toHaveLength(0);
+  });
+});