flex 上下div固定, 中间div自适应

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        html,body{height:100%;margin: 0;padding: 0;}
        .flex-container{height: 100%;display: flex;flex-direction: column;text-align: center;
            width: 100%;
            position: absolute;
            top: 0;
            left:0;
        }
        .flex-item:nth-child(1),.flex-item:nth-child(3){flex-grow: 0;flex-shrink: 0;background-color: #ababab;}
        .flex-item:nth-child(2){flex-grow: 1;flex-shrink: 1;background-color: #000;overflow-y: auto; color: #fff;}
        .center{position: fixed;top: 50%;left: 50%;margin-top: -10px;margin-left: -24px;}
    </style>
</head>
<body>
    <div class="flex-container">
        <div class="flex-item">top</div>
        <div class="flex-item">
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
            center<br>
        </div>
        <div class="flex-item">bottom</div>
    </div>
</body>
</html>