BuddyPress.org

But you haven’t created any rules for sidebar so it can only sit in the flow??!!

It looks as though #sidebar needs to be placed after your #sidebar-content element has closed as you have rules for a classic source order arrangement with #content-sidebar floated 100% width and it’s first child given a right margin to clear space for the sidebar with this arrangement #sidebar element must be flowed after the main sidebar-container closes. Then you need to actually describe sidebar ruleset that floats it left, set a width and an equal negative left margin to match the width.