In window B you set a cookie and in window A you create a setTimeout function that every "x" milliseconds check if there are any new cookie.

Look at this: Javascript communication between browser tabs/windows

You should be able to do this with the keyup event. To be specific, event.target should point at the selected element and event.target.href will give you the href-value of that element. See mdn for more information.

The following code is jQuery, but apart from the boilerplate code, the rest is the same in pure javascript. This is a keyup handler that is bound to every link tag.

$('a').on( 'keyup', function( e ) {
    if( e.which == 9 ) {
        console.log( e.target.href );
    }
} );

jsFiddle: http://jsfiddle.net/4PqUF/

If it has to be a Chrome App controlling contents of a Chrome browser tab, then I think what you suggest at the end is the best option: Chrome app communicates with Chrome extension via message passing and Chrome extension modifies the page.

https://developer.chrome.com/extensions/messaging#external

Probably this does not fit your use case, but if the website could be displayed from the Chrome app, you could use a <webview> tag, the website would not be sandboxed, and you could control its contents using webview.executeScript and the contentload event

https://developer.chrome.com/apps/tags/webview#example https://developer.chrome.com/apps/tags/webview#event-contentload

If I get you correctly, you want to know when a tab/window is effectively closed. Well, AFAIK the only way in JavaScript to detect that is to use either onunload or onbeforeunload events.

Unfortunately (or fortunately?), those events are also fired when you leave a site over a link or your browsers back button. So this is the best answer I can give, I don't think you can natively detect a pure close in JavaScript. Correct me if I'm wrong here.

I got this one working with the beforeunload method

Went some like this:

useEffect(() => {
  window.addEventListener("beforeunload", () => {
    if (chatToEmailServer.chatToServer.length > 0) {
      emailChatHistory();
      setFirstMsg(true);
    }
  });
}, [chatToEmailServer.chatToServer, firstMsg]);

^^^ This is working, but it gives me warning on the code for dependencies.

Trying to fix the dependencies error is something in the backburner because include the emailChatHistory() in the dependencies, then I have to dig all through de code and fix communication with the API and a few other things.... basically it opens another chain of errors that I couldn't figured out.... so, until it breaks, or when I got time, I might change the code later.

Have you tried window.open() in chrome.app.runtime.onLaunched

You can also try and use VisibilityAPI.

document.addEventListener("visibilitychange", function() {
    if (document.hidden){
        console.log("Browser tab is hidden")
    } else {
        console.log("Browser tab is visible")
    }
});

See also here on Stackoverflow (possible duplicate)