解决问题：由于层与下拉框之间的优先级是：下拉框 > 层，因此在显示的时候，会因为优先级的次序而会出现如上问题。（如果几个元素都是层的话，我们可以通过层的 z-index 属性来设置）解决办法就是：给层中放一个优先级比下拉框更高的元素（iframe），从而解决此问题！具体解决代码如下：
<div id="menu" style="position:absolute;
visibility:hidden; top:20px; left:20px;
width:100px; height:200px; background-color:#6699cc;">
<table>
<tr><td>item 1</td></tr>
<tr><td>item 2</td></tr>
<tr><td>item 3</td></tr>
<tr><td>item 4</td></tr>
<tr><td>item 5</td></tr>
</table>
<iframe src="javascript:false" style="position:absolute;
visibility:inherit; top:0px; left:0px; width:100px;
height:200px; z-index:-1; filter='progid:DXImageTransform.Microsoft.Alpha
(style=0,opacity=0)';"></iframe>
</div>
<a href="#" onclick="document.getElementById('menu').
style.visibility='visible'">menu</a>
<form>
<select><option>A form selection list</option></select>
</form>
visibility:hidden; top:20px; left:20px;
width:100px; height:200px; background-color:#6699cc;">
<table>
<tr><td>item 1</td></tr>
<tr><td>item 2</td></tr>
<tr><td>item 3</td></tr>
<tr><td>item 4</td></tr>
<tr><td>item 5</td></tr>
</table>
<iframe src="javascript:false" style="position:absolute;
visibility:inherit; top:0px; left:0px; width:100px;
height:200px; z-index:-1; filter='progid:DXImageTransform.Microsoft.Alpha
(style=0,opacity=0)';"></iframe>
</div>
<a href="#" onclick="document.getElementById('menu').
style.visibility='visible'">menu</a>
<form>
<select><option>A form selection list</option></select>
</form>
