div置顶且屏蔽底下图层的图层

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<script type="text/javascript">
var t_DiglogX,t_DiglogY,t_DiglogW,t_DiglogH;
function DialogShow(showdata,ow,oh,w,h){  //显示DIV置顶层
  var objDialog = document.getElementById("DialogMove");
  if (!objDialog)
  objDialog = document.createElement("div");
  t_DiglogW = ow;
  t_DiglogH = oh;
  DialogLoc();
  objDialog.id = "DialogMove";
  var oS = objDialog.style;
  oS.display = "block";
  oS.top = t_DiglogY + "px";
  oS.left = t_DiglogX + "px";
  oS.margin = "0px";
  oS.padding = "0px";
  oS.width = w + "px";
  oS.height = h + "px";
  oS.position = "absolute";
  oS.zIndex = "5";
  oS.background = "#FFF";
  oS.border = "solid #000 1px";
  objDialog.innerHTML = showdata;
  document.body.appendChild(objDialog);
}
function DialogHide(){  //关闭div置顶层的主调
  ScreenClean();
  var objDialog = document.getElementById("DialogMove");
  if (objDialog)
  objDialog.style.display = "none";
}
function DialogLoc(){  //计算div窗口位置
  var dde = document.documentElement;
  if (window.innerWidth){
  var ww = window.innerWidth;
  var wh = window.innerHeight;
  var bgX = window.pageXOffset;
  var bgY = window.pageYOffset;
  }else{
  var ww = dde.offsetWidth;
  var wh = dde.offsetHeight;
  var bgX = dde.scrollLeft;
  var bgY = dde.scrollTop;
  }
  t_DiglogX = (bgX + ((ww - t_DiglogW)/2));
  t_DiglogY = (bgY + ((wh - t_DiglogH)/2));
}
function ScreenConvert(){  //整个页面区域加屏蔽层
  var objScreen = document.getElementById("ScreenOver");
  if(!objScreen)
  var objScreen = document.createElement("div");
  var oS = objScreen.style;
  objScreen.id = "ScreenOver";
  oS.display = "block";
  oS.top = oS.left = oS.margin = oS.padding = "0px";
  if (document.body.clientHeight) {
  var wh = document.body.clientHeight + "px";
  }else if (window.innerHeight){
  var wh = window.innerHeight + "px";
  }else{
  var wh = "100%";
  }
  oS.width = "100%";
  oS.height = wh;
  oS.position = "absolute";
  oS.zIndex = "3";
  oS.background = "#cccccc";
  oS.filter = "alpha(opacity=50)";
  oS.opacity = 40/100;
  oS.MozOpacity = 40/100;
  document.body.appendChild(objScreen);
  var allselect = document.getElementsByTagName("select");
  for (var i=0; i<allselect.length; i++)
  allselect[i].style.visibility = "hidden";
}
function ScreenClean(){  //清屏
  var objScreen = document.getElementById("ScreenOver");
  if (objScreen)
  objScreen.style.display = "none";
  var allselect = document.getElementsByTagName("select");
  for (var i=0; i<allselect.length; i++)
  allselect[i].style.visibility = "visible";
}
function Demo(string){ //主调
  ScreenConvert();
  var ShowDiv="<div style=\"padding:10px;\">"+string+" <br /><br /><input type=\"button\" onclick=\"DialogHide();\" value=\" 关闭 \"></div>";
  DialogShow(ShowDiv,250,120,300,100);
}
</script>
<style>
body{margin:0}
</style>
<SCRIPT LANGUAGE="JavaScript">
<!--
Demo('________');
//-->
</SCRIPT>