Clients.openWindow()
  The openWindow() method of the Clients
  interface creates a new top level browsing context and loads a given URL. If the calling
  script doesn't have permission to show popups, openWindow() will throw an
  InvalidAccessError.
In Firefox, the method is allowed to show popups only when called as the result of a notification click event.
In Chrome for Android, the method may instead open the URL in an existing browsing context provided by a standalone web app previously added to the user's home screen. As of recently, this also works on Chrome for Windows.
Syntax
self.clients.openWindow(url).then(function(windowClient) {
  // Do something with your WindowClient
});
Parameters
- url
- 
    A USVStringrepresenting the URL of the client you want to open in the window. Generally this value must be a URL from the same origin as the calling script.
Return value
  A Promise that resolves to a WindowClient object if the
  URL is from the same origin as the service worker or a null
  value otherwise.
Examples
// Send notification to OS if applicable
if (self.Notification.permission === 'granted') {
  const notificationObject = {
    body: 'Click here to view your messages.',
    data: { url: self.location.origin + '/some/path' },
    // data: { url: 'http://example.com' },
  };
  self.registration.showNotification('You\'ve got messages!', notificationObject);
}
// Notification click event listener
self.addEventListener('notificationclick', e => {
  // Close the notification popout
  e.notification.close();
  // Get all the Window clients
  e.waitUntil(clients.matchAll({ type: 'window' }).then(clientsArr => {
    // If a Window tab matching the targeted URL already exists, focus that;
    const hadWindowToFocus = clientsArr.some(windowClient => windowClient.url === e.notification.data.url ? (windowClient.focus(), true) : false);
    // Otherwise, open a new tab to the applicable URL and focus it.
    if (!hadWindowToFocus) clients.openWindow(e.notification.data.url).then(windowClient => windowClient ? windowClient.focus() : null);
  }));
});
Specifications
| Specification | 
|---|
| Service Workers 1 # clients-openwindow | 
Browser compatibility
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