<!DOCTYPE html>
<html>
<head>
    <style>
        body {
            margin: 0;
        }
        iframe {
            border: 20px solid blue;
            padding: 12px;
            height: 400px;
            width: 500px;
            margin: 20px;
            background-color: orange;
            outline: 12px solid rgba(0, 0, 0, 0.25);
        }
        .scrollbar-obscurer {
            position: absolute;
            height: 400px;
            width: 18px;
            top: 52px;
            left: 534px;
            background-color: gray;
        }
    </style>
    <script>
        if (window.internals)
            internals.settings.setAsyncFrameScrollingEnabled(true);
    </script>
</head>
<body>
    <iframe srcdoc="
        <style>
        body {
            height: 1000px;
            background-color: silver;
            margin: 10px;
        }
        .box {
            width: 200px;
            height: 200px;
            background-color: green;
        }
        
        </style>
        <body>
            <div class=box></div>
        </body>
    "></iframe>
    <div class="scrollbar-obscurer"></div>
</body>
</html>