test('updates UI after data loads', async () => {
  render(<UserProfile userId="123" />)

  // Wait "long enough" for data to load
  await sleep(2000)

  expect(screen.getByText('Alice')).toBeInTheDocument()
})
// Slow (always waits 2s) and flaky (might not be enough)

test('processes background job', async () => {
  await jobQueue.enqueue({ type: 'send-email', to: 'user@test.com' })

  // Hope the job completes in 5 seconds
  await sleep(5000)

  expect(await getEmailCount('user@test.com')).toBe(1)
})
// Wastes 5s even if job completes in 100ms

test('updates UI after data loads', async () => {
  render(<UserProfile userId="123" />)

  // Wait for specific element to appear
  await waitFor(() => {
    expect(screen.getByText('Alice')).toBeInTheDocument()
  })
})
// Fast: returns as soon as condition is met

test('processes background job', async () => {
  await jobQueue.enqueue({ type: 'send-email', to: 'user@test.com' })

  // Poll for completion with timeout
  await waitFor(
    async () => {
      const count = await getEmailCount('user@test.com')
      expect(count).toBe(1)
    },
    { timeout: 5000, interval: 100 }
  )
})
// Returns immediately when done, fails fast if broken

// For event-based systems
test('receives message after publish', async () => {
  const messagePromise = new Promise(resolve => {
    subscriber.once('message', resolve)
  })

  publisher.publish({ data: 'test' })

  const message = await messagePromise
  expect(message.data).toBe('test')
})