Make the container with the overflow:auto; absolute. http://jsfiddle.net/NicoO/49SY8/
body, html
{
margin:0;
padding:0;
height:100%;
}
div#outer{
position : fixed;
top : 0;
right : 0;
z-index : 200;
width : 320px;
height : 100%;
}
div#middle {
max-width : 300px;
}
div#inner {
overflow: auto;
position: absolute;
bottom:0;
left:0;
right:0;
top:0;
}