In some themes -- or at least the ones that I've tried -- the containing div will not scroll. However, by changing height to some defined value and overflow: auto, it rendered a scrollable and much more useable UI element.

element {
display: block;
z-index: 1006;
outline: 0px none;
position: fixed;
height: 600px;
width: 300px;
top: 0px;
left: 0px;
overflow: auto;
}