这个高度怎么判断

[shao56bamuk]

<style>
.black_overlay{
display: none;
position: absolute;
top: 0%;
left: 0%;
width: 100%;
height: 100%;
background-color: black;
z-index:1001;
-moz-opacity: 0.8;
opacity:.80;
filter: alpha(opacity=80);
}
.white_content {
display: none;
position: absolute;
top: 10%;
left: 10%;
width: 80%;
height: 80%;
border: 16px solid lightblue;
background-color: white;
z-index:1002;
overflow: auto;
}
.white_content_small {
display: none;
position: absolute;
top: 20%;
left: 30%;
width: 40%;
height: 50%;
border: 16px solid lightblue;
background-color: white;
z-index:1002;
overflow: auto;
}
</style>
<script type="text/javascript">
//弹出隐藏层
function ShowDiv(show_div,bg_div){
document.getElementById(show_div).style.display='block';
document.getElementById(bg_div).style.display='block' ;
var bgdiv = document.getElementById(bg_div);
bgdiv.style.width = document.body.scrollWidth;
// bgdiv.style.height = $(document).height();
$("#"+bg_div).height($(document).height());
};
//关闭弹出层
function CloseDiv(show_div,bg_div)
{
document.getElementById(show_div).style.display='none';
document.getElementById(bg_div).style.display='none';
};
</script>

<input id="Button1" type="button" value="点击弹出层" onclick="ShowDiv('MyDiv','fade')" />
<div id="fade" class="black_overlay">
</div>
<div id="MyDiv" class="white_content">
<div style="text-align: right; cursor: default; height: 40px;">
<span style="font-size: 16px;" onclick="CloseDiv('MyDiv','fade')">关闭</span>
</div>
</div>
<p style="height:1800px;">?</p>

==============================

这个遮罩 只能遮住屏幕可见位置 鼠标滚动后 下面无法遮住

请问 .black_overlay{
display: none;
position: absolute;
top: 0%;
left: 0%;
width: 100%;
height: 100%;
background-color: black;
z-index:1001;
-moz-opacity: 0.8;
opacity:.80;
filter: alpha(opacity=80);
}

请问怎么才能全屏遮罩 或者 判断网页整体高度

或者 屏蔽向下滚动

[slwsss]

onscroll 添加这个事件 滚动时重新设置下位置 或者用fixed

[zpjshiwo77]

position:fixed

[似梦飞花]

position:fixed