调试易

例子
<style>
.pad {position:absolute;display:none;z-index:100}
</style><table>
<tr>
 <td onclick="ShowPopMenu('pad1')">pad1</td>
 <td onclick="ShowPopMenu('pad2')">pad2</td>
</tr>
</table>
<span id="pad1" class="pad">
 <iframe src=w2.htm width=100 height=100 frameborder=no style="position:relative;"></iframe>
</span>
<span id="pad2" class="pad">
 <iframe src=about:blank name=popmenu2 width=100 height=100 frameborder=no style="position:relative;"></iframe>
</span><div style="z-index:0">
<select>
<option>aaa
<option>bbb
<option>ccc
</select>
<br>
<iframe src=></iframe>
<span id="view" style="position:absolute;z-index:0"></span>
</div><script>
function test(v) {
  v.innerHTML = "ssss<br><iframe src=w2.htm width=200 height=100 style=\"position:relative\"></iframe>";
}popmenu2.document.write("aaaaaaa<br>ssssssssss<br>ccccccccc<br>tttttttt");function CloseAllPopMenu() {
  var i = 1;
  while(document.all["pad"+i]) {
    document.all["pad"+i].style.display = "none";
    i++;
  }
}function ShowPopMenu(v) {
  var el = event.srcElement;
  v = document.all[v];
  CloseAllPopMenu();
  v.style.left = el.offsetLeft;
  v.style.display = "block";
}</script>

微软有个菜单，你将例子中的 iframe 改成 select 即可演示：
http://www30.brinkster.com/bencalie/test/menu.htm
下载：
http://www30.brinkster.com/bencalie/test/menu.zip