<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
body{
height: 3000px;
}
#modal{
background-color: gray;
position: absolute;
left: 0;
top:0;
border: 1px solid red;
display: none;
width: 300px;
}
</style>
</head>
<body>
<button id="open">window</button>
<div id="modal">Lorem ipsum, dolor sit amet consectetur adipisicing elit. Necessitatibus consequatur vero atque vitae esse iste dicta incidunt aliquid temporibus ex tempora recusandae facere, placeat enim ullam! Consequuntur porro inventore ad!
<button id="close">close</button>
</div>
<script>
document.querySelector('#open').onclick = e => {
document.body.style.overflowY = "hidden";
document.querySelector('#modal').style.display = 'block';
};
document.querySelector('#close').onclick = e => {
document.body.style.overflowY = "scroll";
document.querySelector('#modal').style.display = 'none';
};
</script>
</body>
</html>