<script>
var waitingForClose = true;
window.addEventListener('beforeunload', function() {
    if (waitingForClose)
        alert("FAIL");
}, false);

window.onload = function() {
    // There's no real way to know whether the iframe's attempt to close us
    // actually failed because it would succeed asynchronously and there is no
    // failure event.  The best we can do is wait around for a while.  The one
    // saving grace is that this test is deterministic when it passes.
    window.setTimeout(function() {
        waitingForClose = false;
        opener.done();
    }, 100);
}
</script>
<iframe sandbox="allow-scripts" src="close-top.html"></iframe>