<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>position fixed 测试</title>
</head>
<body>
    <div class="parent bg">
        parent box
        <div class="fixed-box">
            width: 100%;<br/>
            height: 100%;<br/>
            background-color: red;<br/>
            opacity: 0.4;<br/>
            position: fixed;<br/>
            left: 20px;<br/>
            top: 50px;<br/>
        </div>
    </div>
</body>
<style>
    *{
        padding:0;
        margin: 0;
    }
    body{
        width: 100%;
        height: 100vh;
        background-color: #ebe9cc;
        display: flex;
        justify-content: center;
        align-items: center;
    }
    .bg{
        background-color: rgba(89, 89, 89, 0.1);
        backdrop-filter: blur(30px);
        -webkit-backdrop-filter: blur(30px);
        border: 1px solid rgba(255, 255, 255, 0.18);
        box-shadow: rgba(14, 14, 14, 0.19) 0px 6px 15px 0px;
        -webkit-box-shadow: rgba(14, 14, 14, 0.19) 0px 6px 15px 0px;
        border-radius: 4px;
        -webkit-border-radius: 4px;
        /*color: rgba(128, 128, 128, 0.4);*/
    }
    
    .parent{
        width: 80%;
        height: 80%;
        border: 1px solid black;
    }
    .fixed-box{
        width: 100%;
        height: 100%;
        background-color: red;
        opacity: 0.4;
        position: fixed;
        left: 20px;
        top: 50px;
    }
</style>
</html>