What is automatic batching in React 18, how does it differ from React 17 batching, and when would you need to opt out with flushSync?

Imagine you're building with LEGOs and you put several pieces on the board at once instead of one at a time. React 18 does that with updates—it groups them together so the screen doesn't flicker. Sometimes you need to see each piece go down immediately, and that's when you use a special tool called flushSync.