<html>

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

        input {
            width: 100vw;
            height: 100vh;
            font-size: 50vh;
        }
    </style>
    <script type="text/javascript">
        function loaded() {
            document.querySelector("input").focus();
        }

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

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

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

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

</html>