Демонстрационный файл после изменения плагина:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<link rel="stylesheet" type="text/css" href="jScrollPane.css" />
<script type="text/javascript" src="jquery-1.2.6.js"></script>
<script type="text/javascript" src="jScrollPane-1.2.3.js"></script>
<script type="text/javascript">
<!--
$(document).ready(function(){
$('.scroll-pane').jScrollPane({showArrows:true, scrollbarWidth:16, dragMaxHeight:20});
});
//-->
</script>
</head>
<body>
<div id='mes' class="scroll-pane" style='height:200px;width:500px;overflow:auto'></div>
<button onclick="$('#mes').append('<div style=\'border:1px solid green;height:30px\'>test div</div>')">Append</button>
<button onclick="$('.scroll-pane').jScrollPane({showArrows:true, scrollbarWidth:16, dragMaxHeight:20, scrollPosition:$('#mes').get(0).scrollHeight||0});">Scroll</button>
</body>
</html>
Скрипт и CSS взят с
http://code.google.com/p/jscrollpane/downloads/list