index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Minimal Outside</title>
    <style>
        iframe#amos {
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
            width: 100vw;
            height: 100vh;
            margin: 0;
            border: 0;
            padding: 0;
            overflow: hidden;
        }
    </style>
</head>
<body>
    <iframe id="amos"></iframe>
    <script>
        // Callback function to start viewer via viewerAsync
        window.parent = {
            callback: function(viewerWindow, viewerAsync) {
                console.info("initial callback called:", viewerWindow, viewerAsync);
                viewerAsync("../assets/", (api) => {
                    console.info("loaded callback called with API:", api);
                });
            }
        };

        // start src/index.html
        document.getElementById('amos').setAttribute('src', "./src/index.html?onload=window.parent.callback");
    </script>
</body>

</html>