This page demonstrates how to detect if the user has started a download. Instead of employing a polling or server push method, this method simply queries the browser's cookie storage. When the download starts, it sets a cookie, which can be detected by the main page. For this to work, your browser has to satisfy the following three conditions:
- JavaScript execution must be allowed.
- Cookies must be accepted.
- The cookie must be propagated to
document.cookie.
You can visit
https://start-of-download.sauerburger.com/download.php (link)
anytime. There is no JavaScript event attached to the link, you can even
copy+paste the URL and open it yourself. As soon as the download starts,
it sets a cookie. The existence of the cookie is displayed below.
If the panel shows that the download has stated, you can remove the cookie to test it again.
See also my blog about this topic or development repository of this web site.