This appears on Chrome and Safari, but not Firefox. I am on OS X 10.6.latest, running Chrome 29. Colleagues report this on newer versions of OS X too.
When I click on a menu item, a large highlight appears around the entire menu bar. I don't get this on other clickable elements of the page, such as menu blocks, node links, or page title.
The shape is not quite rectangular -- there's a funny blob in the bottom left hand corner, which even more oddly is a different shape on Chrome and Safari.