<html>

<head>
    <style>
        body {
            margin: 0;
        }

        input {
            width: 100%;
            height: 400px;
            margin-bottom: 5000px;
        }
    </style>
    <script type="text/javascript">
        function loaded() {
            var input = document.querySelector("input");
            input.setSelectionRange(0, 0);
            input.focus();
        }

        function focused() {
            setTimeout(() => window.webkit.messageHandlers.testHandler.postMessage("focused"), 0);
        }

        function input() {
            setTimeout(() => window.webkit.messageHandlers.testHandler.postMessage("input"), 0);
        }
    </script>
</head>

<body onload=loaded()>
    <input onfocus=focused() oninput=input()></input>
</body>

</html>