浏览器窗口缩小到宽度小于div模块时，如何让div模块始终显示正中间的内容？

    var docEle = function()
    {
        return document.getElementById(arguments[0]) || false;
    }

    function openNewDiv(_id)
    {
        var m = "mask";
        if (docEle(_id)) document.body.removeChild(docEle(_id));
        if (docEle(m)) document.body.removeChild(docEle(m));

        //mask遮罩层

        var newMask = document.createElement("div");
        newMask.id = m;
        newMask.style.position = "absolute";
        newMask.style.zIndex = "1";
        _scrollWidth = Math.max(document.body.scrollWidth,document.documentElement.scrollWidth);
        _scrollHeight = Math.max(document.body.scrollHeight,document.documentElement.scrollHeight);
        newMask.style.width = _scrollWidth + "px";
        newMask.style.height = _scrollHeight + "px";
        newMask.style.top = "0px";
        newMask.style.left = "0px";
        newMask.style.background = "#33393C";
        newMask.style.filter = "alpha(opacity=40)";
        newMask.style.opacity = "0.40";
        document.body.appendChild(newMask);

        //新弹出层

        var newDiv = document.createElement("div");
        newDiv.id = _id;
        newDiv.style.position = "absolute";
        newDiv.style.zIndex = "9999";
        newDivWidth = 400;
        newDivHeight = 200;
        newDiv.style.width = newDivWidth + "px";
        newDiv.style.height = newDivHeight + "px";
        newDiv.style.top = (document.body.scrollTop + document.body.clientHeight/2 - newDivHeight/2) + "px";
        newDiv.style.left = (document.body.scrollLeft + document.body.clientWidth/2 - newDivWidth/2) + "px";
        newDiv.style.background = "#EFEFEF";
        newDiv.style.border = "1px solid #860001";
        newDiv.style.padding = "5px";
        newDiv.innerHTML = " ";
        document.body.appendChild(newDiv);

        //弹出层滚动居中

        function newDivCenter()
        {
            newDiv.style.top = (document.body.scrollTop + document.body.clientHeight/2 - newDivHeight/2) + "px";
            newDiv.style.left = (document.body.scrollLeft + document.body.clientWidth/2 - newDivWidth/2) + "px";
        }
        if(document.all)
        {
            window.attachEvent("onscroll",newDivCenter);
        }
        else
        {
            window.addEventListener('scroll',newDivCenter,false);
        }

        //关闭新图层和mask遮罩层
        var newA = document.createElement("div");
        newA.innerHTML ="取消";
        var newB = document.createElement("TEXTAREA");
        newB.setAttribute("cols","40");
        newB.setAttribute("rows","10");
        newA.onclick = function(){
            if(document.all)
            {
                window.detachEvent("onscroll",newDivCenter);
            }
            else
            {
                window.removeEventListener('scroll',newDivCenter,false);
            }
            document.body.removeChild(docEle(_id));
            document.body.removeChild(docEle(m));
            return false;
        }
        newDiv.appendChild(newB);
        newDiv.appendChild(newA);
    }


     
    弹出层
    

把图片改成div用css背景方式来显示就可以居中而且不变形。