CSS: Determine if an element has been totally scrolled

Element: scrollHeight property - Web APIs | MDN (mozilla.org)

<!DOCTYPE html>
<html lang="en-US">

<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>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        html,
        body {
            height: 100%;
        }

        div {
            width: 300px;
            height: 500px;
            margin: 30px;
            border: 20px dashed violet;
            padding: 40px;
            background-color: plum;
            position: relative;
            overflow-x: scroll;
            overflow-y: scroll;
            display: flow-root;
        }

        aside {
            width: 100px;
            height: 800px;
            background-color: peru;
            position: absolute;
            margin: 10px;
            padding: 20px;
            border: 30px dotted coral;
            top: 50px;
        }
    </style>
</head>

<body>
    <div>
        <aside></aside>
    </div>
    <script>
        const div = document.querySelector('div')
        const aside = document.querySelector('aside')
        div.onscroll = function() {
            const sum = div.clientHeight + div.scrollTop
            console.log(sum, div.scrollTop)
            console.log(div.scrollHeight - sum)
        }
    </script>
</body>

</html>