Capturing onbeforeunload when page is contained in iframe

I've set up a fiddle to try to verify if the beforeunload event is triggered when the page is used in an iframe.

Since the fiddle show its result in an iframe, I figured it could be easy to verify by just closing the page. I've set up a request bin at pipedream just to see if any requests gets sent, but it doesn't seem to trigger in Chrome.

window.onbeforeunload = function() {
    fetch('https://eoaczcjrpegb7wv.m.pipedream.net')
}

Is is possible to use this event from an iframe or do I need to look in to a different approach?

After a closer look it seem to capture some of the requests. Is this prone to race conditions? If so, are there any more robust alternatives?

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