<!DOCTYPE html>
<html>
<head>
    <style>
        body {
            height: 1000px;
        }
        body.changed {
            height: auto;
        }
        iframe {
            width: 500px;
            height: 400px;
            background-color: #FFFFC1;
            box-shadow: 0 0 10px black;
            border: 2px solid black;
        }
    </style>
    <script>
        window.addEventListener('load', () => {
            requestAnimationFrame(() => {
                document.body.classList.add('changed');
                document.body.offsetWidth;
                parent.subFrameBecameNonScrollable();

                requestAnimationFrame(() => {
                    document.body.classList.remove('changed');
                    document.body.offsetWidth;
                    parent.subFrameBecameScrollable();
                });
            });
        }, false);
    </script>
</head>
<body>
    <p>Middle frame</p>
    <iframe srcdoc="
        <style>
            body { height: 1000px; }
        </style>
        <p>Inner frame</p>">
    </iframe>
</body>
</html>