Overlay-scrollbar is one of those features of the Ubuntu desktop that I find very annoying. I know, I know, some people think that it’s the best thing that’s ever been implemented on the Ubuntu desktop, but…

So one of the first things I tend to do on a new installation of Ubuntu Desktop is to disable it. On previous editions of Ubuntu, that meant uninstalling or removing the liboverlay-scrollbar package.

However, in Ubuntu 13.10, the feature is made possible by the overlay-scrollbar package (liboverlay-scrollbar is not installed). But when I removed the package in a new installation of Ubuntu 13.10, HUD crashed and one particular package that I installed did not load, throwing the error shown in this image.
Firewalld Ubuntu 13.10

So it seems that the overlay-scrollbar package has been tightly integrated into the core of the system, such that removing it will likely cause some system instability. But the overlay scrollbar can be disabled, without uninstalling the overlay-scrollbar package. The best utility I have found that makes it easy to disable the overlay scrollbar is called Ubuntu Tweak. Hint: It can be used for customizing almost every aspect of the desktop. See Ubuntu Tweak: The first app to install on Ubuntu 13.10 for a simple guide on how to install Ubuntu Tweak on Ubuntu 13.10.

With Ubuntu Tweak, you can see the different forms that the scrollbar can take – Normal (Same as Auto), Show Overlay, Never Show Overlay.
Overlay Scrollbar Ubuntu 13.10

The default is the annoying overlay scrollbar.
Ubuntu 13.10 overlay scrollbar

You can disable the overlay and just show a thin red strip to indicate the presence of scrollable content.
Ubuntu 13.10 no overlay scrollbar

But my favorite is the traditional, or normal, scrollbar. I have no idea why anybody thought this was broken and needed to be messed with.
Ubuntu 13.10 normal scrollbar