做出来二级菜单会出现闪烁延迟现象，是怎么回事？
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var aLi=document.getElementsByTagName('li');
for(var i=0; i<aLi.length; i++){
aLi[i].onmouseover=function(){
//鼠标经过一级菜单，二级菜单动画下拉显示出来
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clearInterval(This.time);
This.time=setInterval(function(){
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if(This.offsetHeight>=120){
clearInterval(This.time);
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//鼠标离开菜单，二级菜单动画收缩起来。
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clearInterval(This.time);
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