请问为撒只动两次就不动了？

<div id="demo" onmouseover="clearInterval(timer)" onmouseout="timer=setInterval(mar,30)" style="overflow:hidden; height:250px; width:220px;"> <ul class="icon_1"> <div id="demo1"> <li>00000000000</li> <li>5555555555555</li> <li>9999999999</li> <li>8888888888888</li> <li>7777777777777777777777777777777777777</li> <li>6888888888888888888888888</li> <li>23555555555555555d</li> <li>sddddddddfethhhhhhhhhh</li> <li>546dsafaddddddddddd</li> <li>dsfasdfasdfasdfasdfadsfas</li> </div> <div id="demo2"></div> </ul> </div> <script> var t=getid("demo"),t1=getid("demo1"),t2=getid("demo2"),sh=getid("show"),timer; t2.innerHTML=t1.innerHTML; function mar(){ if(t2.offsetTop<=t.scrollTop) t.scrollTop-=t1.offsetHeight; else t.scrollTop++; } timer=setInterval(mar,30); function getid(id){ return document.getElementById(id); } </script>
 提示：您可以先修改部分代码再运行