div盖不住dropdownlist控件？

在div中放置一个iframe就可以将dropdownlist控件盖住。

例如：

<div id="divMaterial2" runat="server" class="DIVopen"
            style="position:absolute; display:none; z-index:4; top: 174px; left: 409px; width: 400px; height: 300px;" >
          <iframe id="iframe3"  runat="server" style="position:absolute; width:400px; height: 300px;top: 0px; left: 0px; display:none; z-index:-1; filter:Alpha(style=0,opacity=0);" ></iframe>
      <table cellpadding="5" cellspacing="1" >
          <tr class="DIVtableHeader">
              <td>
                    <table style="width: 100%; vertical-align:top;">
                         <tr>

                              <td style="width:20px; vertical-align:top; text-align:right;">
                                    <img alt="关闭" src="../images/Delete.gif" style="cursor:pointer;"
                                        onclick="hideDiv('iframe3');hideDiv('divMaterial2');" /></td>
                          </tr>                           
                        </table>
                        </td>
                </tr>

            </table>
        </div>