Fixed element within a div?

This is a ridiculously basic CSS question, I'm sure, but I'm stumped, so I'm asking for help.

On the following website: (http://www.willmusto.com/demo/tdp/index.php), I want the sidebar (where it says "ABOUT"), which is under the CSS div "sidebar", to be a fixed element, in conjunction with the borders set by the container div. Does that make sense? No matter what I do, the element insists upon being fixed based on the overall width of the browser, and not the container div.

The stylesheet is here: http://www.willmusto.com/demo/tdp/stylesheet.css