<script>
function frameLoaded() {
  if (window.opener)
    window.opener.postMessage('done', '*');
}
</script>
<iframe onload="frameLoaded()"
        src="http://example.test:8080/security/resources/redir.php?url=https://127.0.0.1:8443/security/mixedContent/resources/boring.html"></iframe>