https://culage.hatenablog.com/entry/2019/03/04/000000

http://scrap.php.xdomain.jp/javascript_screen_lock/#i-2

<html>
<head>
<script>
// ロック用関数
function screenLock(){
    // ロック用のdivを生成
    var div = document.createElement("div"); 
    div.style.position = "fixed";
    div.style.left = "0px"; 
    div.style.top = "0px";
    div.style.width = "100%";
    div.style.height = "100%"; 
    div.style.zIndex = "9999";
    div.style.backgroundColor = "black";
    div.style.opacity = 0.8;
    div.style.textAlign = "center";
    div.innerHTML = "<h5 style='color:white;'>アップロード中</h5><img src='loading.gif'>";
 
    var objBody = document.querySelector("body"); 
    objBody.appendChild(div);
 
    setTimeout( function() {
        div.remove();
    }, 3000 );
}
</script>
</head>
<body>
<h1>Javascriptで画面ロックする</h1>
<button onclick="screenLock()">ScreenLock</button>
</body>
</html>