FlexDemo.test.js

import FlexDemo from './FlexDemo';
import { fireEvent, render, screen } from '@testing-library/react';

describe('Demo', () => {
  it('should switch tab when clicking tab title', async () => {
    render(<FlexDemo />);

    expect(screen.queryByText('Panel Three')).toBe(null);
    const tab = screen.getByText('Three');
    fireEvent.mouseDown(tab);
    fireEvent.mouseUp(tab);

    expect(screen.getByText('Panel Three')).toBeVisible();
  });
});