Tabs are hidden when the available space is limited

When using the xwikitabbar CSS class name to organize information in tabs, the last tabs are hidden if the available space for the tab bar is limited. This affects for instance the Main.AllDocs page which has 6 tabs by default, 7 if you enable the link checker too. To reproduce the problem just reduce the size of the browser window (optionally activate both left and right panels).

The simplest solution is to display the tabs on multiple lines. Even if it doesn't look nice, the tab bar is at least functional (all tabs are accessible).