Based on the link you’ve shared, if you want a popup over any desktop application through the browser, use the W3C Notification API. For example
window.onload = function () {
    if (Notification.permission !== "granted")
        Notification.requestPermission();
};
function notifyMe() {
    if (!Notification) {
        alert('Desktop notifications not available in your browser. Try Chromium.'); 
        return;
    }
    if (Notification.permission !== "granted")
        Notification.requestPermission();
    else {
        var notification = new Notification('Notification title', {
            icon: 'Notification icon',
            body: "Notification body",
        });
        notification.onclick = function () {};
    }
}
and call the notifyMe() using the navigator online variable. For example
setInterval(function () {
    if (navigator.onLine)
        notifyMe();
}, 1000);
This will check whether user is connected to internet every 1 second and display a popup if true.
See https://developer.mozilla.org/en/docs/Online_and_offline_events
solved Display an alert visible if the user is using the web or not