Function hideOnClick

packages/react-dom/src/__tests__/ReactDOMFiberAsync-test.js:561–566

()

    function OldApp() {
      const [value, setValue] = React.useState('old');
      function hideOnClick() {
        // Schedule a discrete update.
        setValue('update');
        // Synchronously unmount this root.
        ReactDOM.flushSync(() => oldRoot.unmount());
      }
      return (
        <button onClick={hideOnClick} ref={ref}>
          {value}