<!DOCTYPE html>

<html>
<head>
<style>
    body {
        margin: 0;
    }
    
    .container {
        width: 200px;
        height: 400px;
        outline: 2px solid black;
    }
    
    .box {
        width: 200px;
        height: 200px;
    }

    .sticky {
        position: sticky;
    }
    
    .child {
        position: relative;
        background-color: green;
        z-index: 2;
    }
</style>
</head>
<body>
    <div class="group">
        <div class="container">
            <div class="sticky box">
                <div class="child box"></div>
            </div>
        </div>
    </div>

</body>
</html>