I have a div which is supposed to exactly fill the viewport, using height: 100vh and width: 100%, but (in Chrome and Safari at least), is slightly larger, so that scroll bars are generated.  Why is this and how do I get the div I want?  Example code is:
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8" />
    <title>test</title>
    <style>
        .box {
            height: 100vh;
            width:100%;
            border: 2px solid blue;
            box-sizing: border-box;
        }
    </style>
</head>
<body>
<div class="box"></div>
</body>
</html>