怎样上下切换的时候能让滚动条跟随滚动呢？

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-
transitional.dtd">
<html>
<head>
<title>自动提示的文本框</title>
<style>
<!--
body{
    font-family:Arial, Helvetica, sans-serif;
    font-size:12px; padding:0px; margin:5px;
}
form{padding:0px; margin:0px;}
input{
    /* 用户输入框的样式 */
    font-family:Arial, Helvetica, sans-serif;
    font-size:12px; border:1px solid #000000;
    width:200px; padding:1px; margin:0px;
}
#popup{
    /* 提示框div块的样式 */
    position:absolute; width:202px;
    color:#004a7e; font-size:12px;
    font-family:Arial, Helvetica, sans-serif;
    left:41px; top:25px;
}
#popup.show{
    /* 显示提示框的边框 */    
    border:1px solid #004a7e;
}
#popup.hide{
    /* 隐藏提示框的边框 */
    border:none;
}
/* 提示框的样式风格 */
ul{
    list-style:none;
    margin:0px;
    padding:0px;
    height: 28px;
    overflow: auto;
}
li.mouseOver{
    background-color:#004a7e;
    color:#FFFFFF;
}
li.mouseOut{
    background-color:#FFFFFF;
    color:#004a7e;
}
-->
</style>
<script language="javascript">
var oInputField;    //考虑到很多函数中都要使用
var oPopDiv;        //因此采用全局变量的形式
var oColorsUl;
var aColors = ["red","green","blue","magenta","yellow","chocolate","black","aquamarine","lime","fuchsia","brass","azure","brown","bronze","deeppink","aliceblue","gray","copper","coral","feldspar","orange","orchid","pink","plum","quartz","purple","antiquewith","blanchedalmond","blueviolet","beige","burlywood","bisque","cadetblue","saddlebrown","royalblue","rosybrown","orengered","olivedrab","powderblue","peachpuff","papayawhip","paleturquoise","palevioletred","palegreen","navyblue","navajowhite","palegodenrod","violetred","yellowgreen","tomato","turquoise","thistle","springgreen","steelblue","salmon","scarlet","silver","violet","snow","tan","chartreuse","khaki","mediumslateblue","mediumvioletred","oldlace","maroom","goldenrod","wheat","whitesmoke","moccasin","mistyrose","mintcream","midnightblue","dimgray","darksalmon","slategray","skyblue","sienna","seashell","seagreen","sandybrown","gold","mediumturquoise","navy","mediumspringgreen","mediumseagreen","mediumpurpul","peru","mediumorchid","mediumblue","mediumaquamarine","maroon","limegreen","lightyellow","lightsteelblue","magenta","lightslateblue","lightslategray","lightskyblue","inen","lightseagreen","lightsalmon","lightpink","lightgray","lightgreen","lightgodenrodyellow","indianred","lavender","lightblue","lavenderblush","lightcoral","lightcyan","lightgodenrod","hotpink","greenyellow","lemonchiffon","lawngreen","deepskyblue","honeydew","golenrod","forestgreen","gostwhite","gainsboro","firebrick","dodgerblue","darkturquoise","darkslateblue","darkslategray","darkseagreen","darkred","darkorchid","darkorenge","darkviolet","floralwhite","cyan","darkgray","cornsilk","darkolivegreen","darkgoldenrod","darkblue","darkcyan","darkgreen","darkhaki","ivory","darkmagenta","cornfloewrblue"];
aColors.sort();    //按字母排序，使显示结果更友好
function initVars(){
    //初始化变量
    oInputField = document.forms["myForm1"].colors;
    oPopDiv = document.getElementById("popup");
    oColorsUl = document.getElementById("colors_ul");
}
function clearColors(){
    //清除提示内容
    for(var i=oColorsUl.childNodes.length-1;i>=0;i--)
        oColorsUl.removeChild(oColorsUl.childNodes[i]);
    oPopDiv.className = "hide";
}
function setColors(the_colors){
    //显示提示框，传入的参数即为匹配出来的结果组成的数组
    clearColors();    //每输入一个字母就先清除原先的提示，再继续
    oPopDiv.className = "show";
    var oLi;
    for(var i=0;i<the_colors.length;i++){
        //将匹配的提示结果逐一显示给用户
        oLi = document.createElement("li");
        oColorsUl.appendChild(oLi);
        var tt=document.getElementById('colors').value;
        oLi.trueValue=the_colors[i];
        oLi.colorValue=the_colors[i].replace(tt,"<font color=red>"+tt+"</font>");
        oLi.innerHTML=oLi.colorValue;
        //oLi.appendChild(document.createTextNode(the_colors[i]));        oLi.onmouseover = function(){
            this.className = "mouseOver";    //鼠标经过时高亮
        }
        oLi.onmouseout = function(){
            this.className = "mouseOut";    //离开时恢复原样
        }
        oLi.onclick = function(){
            //用户点击某个匹配项时，设置输入框为该项的值
            oInputField.value = this.trueValue;
            clearColors();    //同时清除提示框
            document.getElementById('colors').onclick=function(){
                location.href='http://www.baidu.com/s?wd='+this.value;
            };
        }
    }
}
var preLi=null;
function findColors(event){
    initVars();        //初始化变量
    var colors_ul = document.getElementById('colors_ul');
    var lis = colors_ul.getElementsByTagName('li');
    var liLen=lis.length;
    var li;
    
    var e = event||window.event;
    if(e.keyCode==38)//上
    {
        if(currentLI>0){
            currentLI--;
        }
        li = lis[currentLI];
        if(preLi!=null){
            preLi.className = "mouseOut";
        }
        li.className="mouseOver";
        preLi=li;
        document.getElementById('colors').value=li.trueValue;
        document.getElementById('colors').onclick=function(){
            location.href='http://www.baidu.com/s?wd=';
        };
    }
    else if(e.keyCode==40)
    {//下
        if(currentLI<liLen-1){
            currentLI++;
        }
        li = lis[currentLI];
        if(preLi!=null){
            preLi.className = "mouseOut";
        }
        li.className="mouseOver";
        preLi=li;
        document.getElementById('colors').value=li.trueValue;
        document.getElementById('colors').onclick=function(){
            location.href='http://www.baidu.com/s?wd='+this.value;
        };
    }else{//其他键值
    
    if(oInputField.value.length > 0){
        var aResult = new Array();        //用于存放匹配结果
        for(var i=0;i<aColors.length;i++)    //从颜色表中找匹配的颜色
            //必须是从单词的开始处匹配
            if(aColors[i].indexOf(oInputField.value) == 0)
                aResult.push(aColors[i]);    //压入结果
        if(aResult.length>0)    //如果有匹配的颜色则显示出来
            setColors(aResult);
        else                    //否则清除，用户多输入一个字母
            clearColors();        //就有可能从有匹配到无，到无的时候需要清除
    }        
    else
        clearColors();    //无输入时清除提示框（例如用户按del键）
    currentLI=-1;
    }
}
</script>
</head>
<body>
<form method="post" name="myForm1">
Color: <input type="text" name="colors" id="colors" onKeyUp="findColors(event);" />
</form>
<div id="popup">
    <ul id="colors_ul"></ul>
</div>
</body>
</html>