<!DOCTYPE HTML>
<html>
<head>
<script>
    const observer = new PerformanceObserver(list => {
        list.getEntries().forEach(entry => {
            window.parent.postMessage(JSON.stringify({type: 'received', entry}), '*');
        });
    });

    observer.observe({entryTypes: ["paint"]});
    window.onmessage = e => {
        window.parent.postMessage(JSON.stringify({supported: PerformanceObserver.supportedEntryTypes, type: 'supported'}), '*');
        document.body.innerText = "TEXT";
    }
</script>
</head>
<body>
</body>
</html>