将滚动条添加到动态ul li

我做了一对调整原始样式表，现在它将显示滚动条，但仅当它超过3个列表项的高度时（在这种情况下为63像素）。这是最终的CSS代码：

I've made a couple of adjustments to your original stylesheet, and now it will show the scroll bar, but only when it exceeds the height of 3 list items (63 pixels in this case). Here's the final CSS code:

#mnav {
    margin-left: -30px;
    margin-right: -30px;
}
#mnav li {
    float: left;
    list-style: none;
    margin:0 10px;/*Keeping 10px space between each nav element*/
}
#mnav li a,/*These can all be merged into a single style*/
#mnav li a:link,
#mnav li a:visited,
#mnav li a:hover,
#mnav li a:active {
    text-decoration: none;
}
#mnav li ul {
    display: none;/*This is the default state.*/
    z-index: 9999;
    position: absolute;
    width: 400px;
    max-height:63px;/*The important part*/
    overflow-y:auto;/*Also...*/
    overflow-x:hidden;/*And the end of the important part*/
    margin: 0px;
    padding-left:5px;/*Removing the large whitespace to the left of list items*/
    border: 1px solid #ddd;
}
#mnav li:hover ul, #mnav li.sfhover ul {
    display:block;/*This is the hovered state*/
}
#mnav li ul li a, #mnav li ul li a:link, #mnav li ul li a:visited {
    display: block;
    margin: 0;
    text-decoration: none;
    z-index: 9999;
    border-bottom: 1px dotted #ccc;
    width:400px;
}
#mnav li ul li a:hover, #mnav li ul li a:active {
    display: block;
    margin: 0;
    text-decoration: none;
}

以下是它的演示 。出于演示目的，我还要在主页和 SQL Server与Oracle 中插入2个额外的< li> 元素强>物品。如果列表项少于3个， Home 项将显示弹出窗口的行为。

Here's the demo of it. I've also, for demonstration purposes, inserted 2 extra <li> elements to the Home and SQL Server vs Oracle items. The Home item will show how the popup behaves if there are fewer than 3 list items there.

要解释每个更改的位，首先实际上诀窍的代码：

To explain each of the changed bits, first the code that actually does the trick:

• 将 max-height 定义为63如果它保持低于63px高，将使ul表现正常，但如果超过该值，它将溢出并显示滚动条。如果您想显示更多项目，只需将 max-height 增加到所需的高度。目前每个项目的高度为21px，所以我用它来获得3个项目的63px。


• 由于溢出应该只显示垂直方向的滚动条，只有 overflow-y：auto; 应该在那里，并且 overflow-x：hidden 以防止水平方向滚动条。

• defining the max-height to 63 will make the ul behave normally if it stays under 63px high, but if it exceeds that, it will overflow and show a scroll bar. If you want to show more items, just increase the max-height to the desired height. Currently the height of each item is 21px, so I used that to get the 63px for 3 items.
• Since the overflow should only show a scroll bar for the vertical direction, only overflow-y:auto; should be there, and overflow-x:hidden to prevent a scrollbar in the horizontal direction.

然后我做的其他一般性更改：

And then the other general changes I made:

• 我在项目之间添加了20px的边距（元素两边各10px），使列表在这里看起来更好一些。你可能想要应用自己的风格，这只适用于这个演示。


• 我已经将你的隐藏技术改为推迟到 -999em 向左通过隐藏它：display 。这样做更好，因为 display：none 的元素不会在搜索引擎中呈现，所以这将有助于这些情况。一般来说，我认为隐藏 display：none 的东西比把它推出屏幕更好，而它实际上仍然存在


• I已经删除了 ul 左边的填充，因为它看起来很糟糕。如果您没有将它用于虚线/编号列表，则无需使用默认填充。

• I've added 20px margin between items (10px on either side of the element) to make the list look a bit better here. You may want to apply your own style though, this is just for this demo.
• I've changed your hiding technique from 'shoving it off to -999em to the left' to hiding it via display:none. This is better to work with, since elements that are display:none will not render in search engines, so this will help in those situations. In general I think hiding things with display:none is just better than shoving it off the screen, while it's still actually there
• I've removed the padding to the left of the ul since it just looked quite bad. No need for the default padding if you're not using it for a dotted/numbered list.

考虑到您的评论，这也应该有效 Zachary Carter 的回答，因为如果你将 max-height 定义为210px（10项），这将不会显示一个巨大的白框。

This should also work considering your comment to Zachary Carter's answer, since this won't show a huge white box if you define the max-height to 210px (10 items).