How about just dropping/setting the line-height for the navmenu to be quite close when it pushes to a second line (as theres no need for a large line-height like it currently has) and then just a margin-top on the floatcontainer below. It's far from elegant but it will mean that any menu that gets pushed down is still visible and clickable as it sits in the margin.
Even just padding-top on the floatcontainer would do similar and make it usable at least.