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                <div id="root"></div>


                body {
  font-family: -apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,"Helvetica Neue",sans-serif;
  font-size: 20px;
  background: #1e1f20;
  color: #eee;

h1 {
  font-weight: 400;

button {
  padding: 8px 16px;
  background: crimson;
  color: white;
  border: none;
  font-size: inherit;


                function copyStyles(sourceDoc, targetDoc) {
  Array.from(sourceDoc.styleSheets).forEach(styleSheet => {
    if (styleSheet.cssRules) { // true for inline styles
      const newStyleEl = sourceDoc.createElement('style');

      Array.from(styleSheet.cssRules).forEach(cssRule => {

    } else if (styleSheet.href) { // true for stylesheets loaded from a URL
      const newLinkEl = sourceDoc.createElement('link');

      newLinkEl.rel = 'stylesheet';
      newLinkEl.href = styleSheet.href;

class MyWindowPortal extends React.PureComponent {
  constructor(props) {
    this.containerEl = document.createElement('div'); // STEP 1: create an empty div
    this.externalWindow = null;

  componentDidMount() {
    // STEP 3: open a new browser window and store a reference to it
    this.externalWindow ='', '', 'width=600,height=400,left=200,top=200');

    // STEP 4: append the container <div> (that has props.children appended to it) to the body of the new window
    this.externalWindow.document.title = 'A React portal window';
    copyStyles(document, this.externalWindow.document);
    // update the state in the parent component if the user closes the 
    // new window
    this.externalWindow.addEventListener('beforeunload', () => {

  componentWillUnmount() {
    // This will fire when this.state.showWindowPortal in the parent component becomes false
    // So we tidy up by just closing the window
  render() {
    // STEP 2: append props.children to the container <div> that isn't mounted anywhere yet
    return ReactDOM.createPortal(this.props.children, this.containerEl);

class App extends React.PureComponent {
  constructor(props) {
    this.state = {
      counter: 0,
      showWindowPortal: false,
    this.toggleWindowPortal = this.toggleWindowPortal.bind(this);
    this.closeWindowPortal = this.closeWindowPortal.bind(this);

  componentDidMount() {
    window.addEventListener('beforeunload', () => {
    window.setInterval(() => {
      this.setState(state => ({
        counter: state.counter + 1,
    }, 1000);
  toggleWindowPortal() {
    this.setState(state => ({
      showWindowPortal: !state.showWindowPortal,
  closeWindowPortal() {
    this.setState({ showWindowPortal: false })
  render() {
    return (
        <h1>Counter: {this.state.counter}</h1>
        <button onClick={this.toggleWindowPortal}>
          {this.state.showWindowPortal ? 'Close the' : 'Open a'} Portal
        {this.state.showWindowPortal && (
          <MyWindowPortal closeWindowPortal={this.closeWindowPortal} >
            <h1>Counter in a portal: {this.state.counter}</h1>
            <p>Even though I render in a different window, I share state!</p>
            <button onClick={() => this.closeWindowPortal()} >
              Close me!

ReactDOM.render(<App/>, document.getElementById('root'));