Handling Server Rendering

πŸ‘¨β€πŸ’Ό We don't currently do any server rendering, but in the future we may want to and this requires some special handling with useSyncExternalStore.
πŸ§β€β™‚οΈ I've simulated a server rendering environment by adding some code to the bottom of our file. First, we render the <App /> to a string, then we set that to the innerHTML of our rootEl. Then we call hydrateRoot to rehydrate our application.
const rootEl = document.createElement('div')
document.body.append(rootEl)
// simulate server rendering
rootEl.innerHTML = (await import('react-dom/server')).renderToString(<App />)

// simulate taking a while for the JS to load...
await new Promise((resolve) => setTimeout(resolve, 1000))

ReactDOM.hydrateRoot(rootEl, <App />)
πŸ‘¨β€πŸ’Ό This is a bit of a hack, but it's a good way to simulate server rendering and ensure that our application works in a server rendering situation.
Because the server won't know whether a media query matches, we can't use the getServerSnapshot() argument of useSyncExternalStore. Instead, we'll leave that argument off, and wrap our <NarrowScreenNotifier /> in a <Suspense /> component with a fallback of "" (we won't show anything until the client hydrates).
With this, you'll notice there's an error in the console. Nothing's technically wrong, but React logs this in this situation (I honestly personally disagree that they should do this, but πŸ€·β€β™‚οΈ). So as extra credit, you can add an onRecoverableError function to the hydrateRoot call and if the given error includes the string 'Missing getServerSnapshot' then you can return, otherwise, log the error.
Good luck!
import { hydrateRoot } from 'react-dom/client'

const root = hydrateRoot(document.getElementById('root'), <App />, {
	onRecoverableError: (error, errorInfo) => {
		console.error('Caught error', error, error.cause, errorInfo.componentStack)
	},
})