js下拉菜单如何才能不被select等form标记挡住

要在菜单层下建一个同样大小的iframe.

<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>