The dockbar becomes transparent when you switch between pages.
steps to reproduce.
1) add new page called test
2) on test page, add web content display portlet
3) add new page called test2
3) add web content display portlet on page test2
4) go back to test page. top menu will 'hide' behind Liferay Logo
Also, when the dockbar is transparent and you click on Toggle Edit Controls, the top menu bar itself will become unaccessible.
no exceptions were thrown during this process.
i tested this on geronimo, but it occurs across other app servers as well.