我想让盒子滑动的时候才在最上面图层怎么做

img

img


我想让盒子滑动的时候才在最上面图层，不滑动屏幕的时候，它保持在最下面图层


 
<!DOCTYPE html>
<html>
 
    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1,maximum-scale=1,user-scalable=no" />
        <title>导航栏吸顶效果</title>
        <style type="text/css">
            html, body {
                width: 100%;
                min-width: 320px;
                max-width: 750px;
                margin: 0 auto;
                background: #FAFBFC;
                color: #fff;
                font: normal 14px/1.5 "Helvetica Neue", "Hiragino Sans GB", "Microsoft YaHei", "\9ED1\4F53", Arial, sans-serif;
            }
            .top{background-color: greenyellow;height: 50px; display: none;position: fixed;width:100%; top:0;left:0;}
            .top.fixed{display: block}
            .nav {
                line-height: 44px;
                background-color: red;
            }
            .list {
                background-color: blue;
                height: 1200px;
                ;
            }
            .fixed{position: fixed;top: 0;left: 0;right: 0;}
        </style>
    </head>
 
    <body>
        <div class="top" id="nav_model">我是隐藏的导航栏</div>
        <div class="nav">我是显示的导航栏</div>
        <div class="list">
            <div>我是列表内容1</div>
            <div>我是列表内容1</div>
            <div>我是列表内容1</div>
            <div>我是列表内容1</div>
            <div>我是列表内容1</div>
            <div>我是列表内容1</div>
            <div>我是列表内容1</div>
        </div>
        <script>
            //获取滚动条当前的位置
            function getScrollTop() {
                var scrollTop = 0;
                if(document.documentElement && document.documentElement.scrollTop) {
                    scrollTop = document.documentElement.scrollTop;
                } else if(document.body) {
                    scrollTop = document.body.scrollTop;
                }
                return scrollTop;
            }
            //获取导航栏的dom节点
            var nav_dom=document.getElementById("nav_model");
            //监听滚动条
            window.onscroll = function() {
                //判断滚动条大于30,导航栏吸顶
                if(getScrollTop() > 50) {
                    nav_dom.className = "top fixed";
                } else {
                    nav_dom.className = "top";
                }
            }
        </script>
    </body>
 
</html>