Stock, default page layout – div.menu

My test wp installation at https://word.patch.com shows the problem.
When I add content to the header section and it grows wide the rightmost section disappears underneath the menu div.
I know this is not CSS support central, pointers to a primer on this kind of layout issue would be helpful. My googling is failing me at the moment.
Of course, a description of how to mod the css or index.php to allieviate the problem would be welcome.