Implement a class-based <ErrorBoundary> that catches render errors in its subtree and shows a fallback. Wrap a <Crashy> child with it.

Name three kinds of errors an <ErrorBoundary> does NOT catch. How do you handle each one?

When <App /> is wrapped in <React.StrictMode>, your useEffect setup logs run twice in development. Why, and what does this catch?

The effect below opens a websocket but never closes it. Under StrictMode the room shows two listeners after mount. Fix the effect.