不知道为什么拖动没有反应，求大神解答

Question

// JavaScript Documentfunction getByClass(ClassName,parent){ var oParent=parent?document.getElementById(parent):document, eles=[], elements=oParent.getElementsByTagName("*"); for(var i=0,l=elements.length;i<l;i++){ if(elements[i].className=ClassName){ eles.push(elements[i]); }}return eles;}window.onload=drag;function drag(){ var oTitle=document.getElementsByTagName("login_logo_webqq","loginPanel")[0]; oTitle.onmousedown=fnDown();}function fnDown(){ var oDrag = document.getElementById("loginPanel"); document.onmousemove=function(event){ event=event||window.event; oDrag.style.left=clientX+"px"; oDrag.style.top=clientY+"px"; }}