You suggested border container, that is something different. if you use dojo layout, in browser it fills only visible screen area and scrolls content inside dojo containers. What I have in mind is scrollable content longer than browser window, and panel which sticks to the edge (left/right/top/bottom) of browser window.
I know it is just matter of JS and CSS, but it would be nice to have it based on simple extlib control.
Something like: http://spyrestudios.com/demos/sliding-panel-fixed