Large number of tabs breaking Tapestry?

I'm building a site for a client with Tapestry, and am finding that, if (for instance) a user page has a large number of tabs, the page rendering is broken: it insists on laying out the LI tags all the way out to the right, exceeding the width of the mainContent area and falling underneath the right sidebar. (See the attached image.)

Has anyone else seen this, and is there anything to be done about it? I've tried various tweaks to the CSS involving tabs, but haven't come across anything that works.