html {
box-sizing: border-box;
}
*, *:before, *:after {
box-sizing: inherit;
}
* {
margin: 0;
padding: 0;
}
body {
line-height: 1.5;
overflow: hidden;
}
main {
width: 420px;
background: silver;
overflow: auto;
}<!DOCTYPE html>
<html lang="ru">
<head>
<meta name="viewport" content="width=device-width,initial-scale=1.0">
<title>artefact</title>
<link rel="stylesheet" type="text/css" href="popup1.css"/>
</head>
<body>
<main>
<p>0</p><p>1</p><p>2</p><p>3</p><p>4</p><p>5</p><p>6</p><p>7</p><p>8</p><p>9</p><p>10</p>
</main>
</body>
</html>
А реальное окошко - оно же не такое.