下拉框处于漂浮层上面的问题解决方案

由于<select>的优先级高于<div>  在IE6下容易出现漂浮层被下拉框覆盖的情况，IE8下未出现此类问题。

下面是页面代码：

<div id='img' style='position:absolute;z-index:9999999;' onmouseover='mystop()' onmouseout= 'start()' runat="server">
<table border="0" cellpadding="0" cellspacing="0">
  <tr>
    <td>&nbsp;</td>
    <td align="right"><img src="images/chacha.jpg" onclick="javascript:document.getElementByIdx('img').style.display='none'" style="cursor:hand"/></td>
  </tr>
  <tr>
    <td colspan="2"><asp:Literal ID="ads" runat="server"></asp:Literal></td>
  </tr>
</table>
<iframe src="" frameborder="0" style="filter:chroma(color=#ffffff);position:absolute;visibility:inherit;top:0px;left:0px;expression_r(this.parentNode.offsetWidth);height:expression_r(this.parentNode.offsetHeight);z-index:-1;filter='progid:DXImageTransform.Microsoft.Alpha(style=0,opacity=0)';"></iframe>
</div>
<script language='javascript'>
var xPos = 800;var yPos = 0; var step = 1;var delay = 10;var height = 0; var Hoffset = 0;var Woffset = 0; var yon = 0;var xon = 0; var xon = 0; var interval;
var img = document.getElementByIdx('img');
img.style.top = 0;
function changePos(){
width = document.body.clientWidth;
height = document.body.clientHeight;
Hoffset = img.offsetHeight;
Woffset = img.offsetWidth;
img.style.left = xPos + document.body.scrollLeft;
img.style.top = yPos + document.body.scrollTop;
if (yon) {
yPos = yPos + step;
}else {
yPos = yPos - step;
}
if (yPos < 0) {
yon = 1;
yPos = 0;
}
if (yPos >= (height - Hoffset)) {
yon = 0;
yPos = (height - Hoffset);
}
if (xon) {
xPos = xPos + step;
}else {
xPos = xPos - step;
}
if (xPos < 0) {
xon = 1;
xPos = 0;
}
if (xPos >= (width - Woffset)) {
xon = 0;
xPos = (width - Woffset);
}
}
function start() {
img.visibility = 'visible';
interval = setInterval('changePos()', delay);
}
function mystop(){
clearInterval(interval)
}
 start()
</script>

上面的代码在IE6下测试过页面上包含的所有下拉框都处在这个漂浮层的下面。

其中的：

<iframe src="" frameborder="0" style="filter:chroma(color=#ffffff);position:absolute;visibility:inherit;top:0px;left:0px;expression_r(this.parentNode.offsetWidth);height:expression_r(this.parentNode.offsetHeight);z-index:-1;filter='progid:DXImageTransform.Microsoft.Alpha(style=0,opacity=0)';"></iframe>

是最关键的。