sudo aptitude purge liboverlay-scrollbar-0.1-0 liboverlay-scrollbar-0.2-0 liboverlay-scrollbar3-0.2-0 overlay-scrollbar
Clearly overlay-scrollbar is screwed up. Oddly, this was removed from Linux Mint 12, but somehow snuck back into Linux Mint 13.
Thanks to MartinVW and LewRockwellFAN at http://forums.mate-desktop.org/viewtopic.php?f=2&t=113&p=1422
As an aside, it appears that this scrollbar crap is intentional. The following is from a posting elsewhere on "Ayatana scrollbars":
Overlay Scrollbar – The overlay scrollbar, or the Ayatana Scrollbar, is a feature designed to solve a non-existent problem. According to the official description, it was designed to “improve the user’s ability to focus on content and applications” and to “ensure that scrollbars take up no active screen real-estate” thereby “reducing the waste of space and distracting clutter that a traditional scrollbar entails.” That is pure nonsense. It just creates more problems than it solves. In fact, it does not solve any problem, because as stated earlier, there is no problem to solve, as far the scrollbar is concerned.
Aside from making you “look” for the scrollbar before you can use it, it creates an inconsistency in the system because some applications, like Firefox, will have the traditional scrollbar, while native Ubuntu applications will have the overlay scrollbar.