CEP javascript won't execute when launching the extension from menu

<!doctype html>
<html lang="en">
<head>
<title>**</title>
</head>
<body>
</body>
<script>
window.onload = function(){
alert("hiloo");
}
</script>
</html>

alert won't  execute unless I set UI in manifest.xml as
<Type>Modeless</Type>

..but I want type to be Panel
what am I missing?