I have been working on a project and have used the Abound theme. The side bar menu is Accordian Menu and the top is purely css. The left sidebar (fixed width) scrolls separately from the main content (fluid) and vice versa (last screen shot).
The problem is that it only works in Firefox (might work in others but I have only used it in FF). My only requirement was for it to work in FF but, personally I would like to make it work in as many browsers as possible.
I have modified the menu (code and css) and abound theme heavily as you can see in the screen shots below. I don't have that much time right now to convert it.
I know it definitely doesn't work in IE. If anyone would like to help convert it I'll send it to you and we could post it for others to use once it works fully.