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chromium/third_party/blink/web_tests/http/tests/devtools/sources/debugger/resources/child.html 

<body>
    <div>Child Frame</div>
    <button onclick="sendRequest()">send sync xhr</button>
<script>
function sendRequest() {
    var xhr = new XMLHttpRequest();
    xhr.open('POST', './resource/unreachable', false);
    function onXhrLoad()
    {
        if (!parent)
            return;
        parent.removeIframe();
        debugger; // should not stop here
    }
    xhr.addEventListener('load', onXhrLoad);
    xhr.send('foo');
}
</script>