mac os x 关闭自动隐藏滚动条功能

添加样式即可

::-webkit-scrollbar {
    -webkit-appearance: none;
    width: 7px;
}
::-webkit-scrollbar-thumb {
    border-radius: 4px;
    background-color: rgba(0,0,0,.5);
    box-shadow: 0 0 1px rgba(255,255,255,.5);
}

见

http://simurai.com/blog/2011/07/26/webkit-scrollbar/

http://stackoverflow.com/questions/7492062/css-overflow-scroll-always-show-vertical-scroll-bar