BuddyPress.org

If still having issues you can try a few thngs, firstly as a test add zoom:1; to #content .padder to establish whether .padder requires hasLayout..

Go to a more traditional source order set of styles which would involve giving the #content element a negative left margin equal or greater than the sidebar then padder gets the a normal margin to pull it clear of the sidebar area, the sidebar is floated and has a negative margin 100%.