Div的大小控制

我在页面上定了了一个div，希望在表单提交后这个div能够显示出来，并占满整个页面，可以当页面往下拉的时候发现div没有随着遮盖，请问该如何才能够让这个div把页面遮住，并且页面往下拉的时候照样能够遮住？急啊……

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LZ网上找javascript的遮罩层有一大堆呢
楼主看一下这篇文章，详细介绍了遮罩层的原理和实现：
http://www.cnblogs.com/cloudgamer/archive/2008/09/15/1290954.html测试代码：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>JavaScript 仿LightBox内容显示效果</title>
</head><body><br /><br /><br /><br /><script>var isIE = (document.all) ? true : false;var isIE6 = isIE && ([/MSIE (\d)\.0/i.exec(navigator.userAgent)][0][1] == 6);var $ = function (id) {
return "string" == typeof id ? document.getElementById(id) : id;
};var Class = {
create: function() {
return function() { this.initialize.apply(this, arguments); }
}
}var Extend = function(destination, source) {
for (var property in source) {
destination[property] = source[property];
}
}var Bind = function(object, fun) {
return function() {
return fun.apply(object, arguments);
}
}var Each = function(list, fun){
for (var i = 0, len = list.length; i < len; i++) { fun(list[i], i); }
};var Contains = function(a, b){
return a.contains ? a != b && a.contains(b) : !!(a.compareDocumentPosition(b) & 16);
}
var OverLay = Class.create();
OverLay.prototype = {
  initialize: function(options) { this.SetOptions(options);

this.Lay = $(this.options.Lay) || document.body.insertBefore(document.createElement("div"), document.body.childNodes[0]);

this.Color = this.options.Color;
this.Opacity = parseInt(this.options.Opacity);
this.zIndex = parseInt(this.options.zIndex);

with(this.Lay.style){ display = "none"; zIndex = this.zIndex; left = top = 0; position = "fixed"; width = height = "100%"; }

if(isIE6){
this.Lay.style.position = "absolute";
//ie6设置覆盖层大小程序
this._resize = Bind(this, function(){
this.Lay.style.width = Math.max(document.documentElement.scrollWidth, document.documentElement.clientWidth) + "px";
this.Lay.style.height = Math.max(document.documentElement.scrollHeight, document.documentElement.clientHeight) + "px";
});
//遮盖select
this.Lay.innerHTML = '<iframe style="position:absolute;top:0;left:0;width:100%;height:100%;filter:alpha(opacity=0);"></iframe>'
}
  },
  //设置默认属性
  SetOptions: function(options) {
    this.options = {//默认值
Lay: null,//覆盖层对象
Color: "#fff",//背景色
Opacity: 50,//透明度(0-100)
zIndex: 1000//层叠顺序
    };
    Extend(this.options, options || {});
  },
  //显示
  Show: function() {
//兼容ie6
if(isIE6){ this._resize(); window.attachEvent("onresize", this._resize); }
//设置样式
with(this.Lay.style){
//设置透明度
isIE ? filter = "alpha(opacity:" + this.Opacity + ")" : opacity = this.Opacity / 100;
backgroundColor = this.Color; display = "block";
}
  },
  //关闭
  Close: function() {
this.Lay.style.display = "none";
if(isIE6){ window.detachEvent("onresize", this._resize); }
  }
};var LightBox = Class.create();
LightBox.prototype = {
  initialize: function(box, options) {

this.Box = $(box);//显示层

this.OverLay = new OverLay(options);//覆盖层

this.SetOptions(options);

this.Fixed = !!this.options.Fixed;
this.Over = !!this.options.Over;
this.Center = !!this.options.Center;
this.onShow = this.options.onShow;

this.Box.style.zIndex = this.OverLay.zIndex + 1;
this.Box.style.display = "none";

//兼容ie6用的属性
if(isIE6){
this._top = this._left = 0; this._select = [];
this._fixed = Bind(this, function(){ this.Center ? this.SetCenter() : this.SetFixed(); });
}
  },
  //设置默认属性
  SetOptions: function(options) {
    this.options = {//默认值
Over: true,//是否显示覆盖层
Fixed: false,//是否固定定位
Center: false,//是否居中
onShow: function(){}//显示时执行
};
    Extend(this.options, options || {});
  },
  //兼容ie6的固定定位程序
  SetFixed: function(){
this.Box.style.top = document.documentElement.scrollTop - this._top + this.Box.offsetTop + "px";
this.Box.style.left = document.documentElement.scrollLeft - this._left + this.Box.offsetLeft + "px";

this._top = document.documentElement.scrollTop; this._left = document.documentElement.scrollLeft;
  },
  //兼容ie6的居中定位程序
  SetCenter: function(){
this.Box.style.marginTop = document.documentElement.scrollTop - this.Box.offsetHeight / 2 + "px";
this.Box.style.marginLeft = document.documentElement.scrollLeft - this.Box.offsetWidth / 2 + "px";
  },
  //显示
  Show: function(options) {
//固定定位
this.Box.style.position = this.Fixed && !isIE6 ? "fixed" : "absolute"; //覆盖层
this.Over && this.OverLay.Show();

this.Box.style.display = "block";

//居中
if(this.Center){
this.Box.style.top = this.Box.style.left = "50%";
//设置margin
if(this.Fixed){
this.Box.style.marginTop = - this.Box.offsetHeight / 2 + "px";
this.Box.style.marginLeft = - this.Box.offsetWidth / 2 + "px";
}else{
this.SetCenter();
}
}

//兼容ie6
if(isIE6){
if(!this.Over){
//没有覆盖层ie6需要把不在Box上的select隐藏
this._select.length = 0;
Each(document.getElementsByTagName("select"), Bind(this, function(o){
if(!Contains(this.Box, o)){ o.style.visibility = "hidden"; this._select.push(o); }
}))
}
//设置显示位置
this.Center ? this.SetCenter() : this.Fixed && this.SetFixed();
//设置定位
this.Fixed && window.attachEvent("onscroll", this._fixed);
}

this.onShow();
  },
  //关闭
  Close: function() {
this.Box.style.display = "none";
this.OverLay.Close();
if(isIE6){
window.detachEvent("onscroll", this._fixed);
Each(this._select, function(o){ o.style.visibility = "visible"; });
}
  }
};</script>
<style>
.lightbox{width:300px;background:#FFFFFF;border:1px solid #ccc;line-height:25px; top:20%; left:20%;}
.lightbox dt{background:#f4f4f4; padding:5px;}
</style><dl id="idBox" class="lightbox">
  <dt id="idBoxHead"><b>LightBox</b> </dt>
  <dd>
    内容显示
    <br /><br />
    <input name="" type="button" value=" 关闭 " id="idBoxCancel" />
    <br /><br />
  </dd>
</dl>
<div style="margin:0 auto; width:900px; height:1000px; border:1px solid #000000;"><input type="button" value="关闭覆盖层" id="btnOverlay" />
<input type="button" value="黑色覆盖层" id="btnOverColor" />
<input type="button" value="全透覆盖层" id="btnOverOpacity" />
<input type="button" value="定位效果" id="btnFixed" />
<input type="button" value="居中效果" id="btnCenter" />
<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />
<select>
<option>覆盖select测试</option>
</select>
<input name="" type="button" value=" 打开 " id="idBoxOpen" /></div><script>var box = new LightBox("idBox");$("idBoxCancel").onclick = function(){ box.Close(); }
$("idBoxOpen").onclick = function(){ box.Show(); }$("btnOverlay").onclick = function(){
box.Close();
if(box.Over){
box.Over = false;
this.value = "打开覆盖层";
} else {
box.Over = true;
this.value = "关闭覆盖层";
}
}$("btnOverColor").onclick = function(){
box.Close();
box.Over = true;
if(box.OverLay.Color == "#fff"){
box.OverLay.Color = "#000";
this.value = "白色覆盖层";
} else {
box.OverLay.Color = "#fff"
this.value = "黑色覆盖层";
}
}$("btnOverOpacity").onclick = function(){
box.Close();
box.Over = true;
if(box.OverLay.Opacity == 0){
box.OverLay.Opacity = 50;
this.value = "全透覆盖层";
} else {
box.OverLay.Opacity = 0;
this.value = "半透覆盖层";
}
}$("btnFixed").onclick = function(){
box.Close();
if(box.Fixed){
box.Fixed = false;
this.value = "定位效果";
} else {
box.Fixed = true;
this.value = "取消定位";
}
}$("btnCenter").onclick = function(){
box.Close();
if(box.Center){
box.Center = false;
box.Box.style.left = box.Box.style.top = "20%";
box.Box.style.marginTop = box.Box.style.marginLeft = "0";
this.value = "居中效果";
} else {
box.Center = true;
this.value = "重新定位";
}
}
</script></body>
</html>