当<menu>元素在HTML中显示为上下文菜单时，将触发onshow事件。
当<menu>元素显示为上下文菜单时，您可以尝试运行以下代码来执行脚本-
<!Doctype html>
<html>
<head>
<title>HTML menu</title>
</head>
<body>
<div style = "border:1px solid #000; padding:20px;" contextmenu = "clickmenu">
<p>Right click inside here....</p>
<menu type = "context" id = "clickmenu" onshow="display()">
<menuitem label = "Nhooo" onclick = ""></menuitem>
<menuitem label = "Tutorials Library" onclick = ""></menuitem>
<menuitem label = "Coding Ground" onclick = ""></menuitem>
<menuitem label = "Q/A" onclick = ""></menuitem>
</menu>
</div>
<script>
function display() {
alert("上下文菜单现在将可见。");
}
</script>
</body>
</html>