position: fixed 为什么在MAC机上不能用？

promptBox.show();
promptBox.html('推荐成功！');
promptBox.fadeOut(1500);

#promptBox {
    position: fixed;
    margin: 0 auto;
    
    top: 50%;
    left: 45%;
    width: 300px;
    height: 50px;
    border-radius: 5px;
    background: #777;
    border: 1px solid #ccc;
    color: #FFFFFF;
    padding: 14px 22px;
    font-size: 14px;
    font-family: "微软雅黑";
    font-weight: normal;
    color: white !important;
    opacity: .8;
    z-index: 10001;
    
}

在MAC机上的效果是position: fixed; background: #777;这个不起作用了。是用谷歌浏览器测试的

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http://raowensheng.com/2015/08/23/%E8%8B%B9%E6%9E%9C%E6%89%8B%E6%9C%BAios%E7%B3%BB%E7%BB%9F%E7%9A%84%E6%B5%8F%E8%A7%88%E5%99%A8input%E7%84%A6%E7%82%B9%E7%8A%B6%E6%80%81%E5%AF%B9positionfixed%E4%B8%8D%E5%85%BC%E5%AE%B9%E7%9A%84/