A new version of overlay scrollbars has been released bringing in smart edge detection behavior, new orange highlight around the thumb and many other fixes.
The thumb is now shown in middle of the vertical bar when a window is too close to the screen edge but not actually touching it.
You can install these scrollbars from a PPA that will provide milestone updates for Natty users to test. Run the following commands:
sudo add-apt-repository ppa:ayatana-scrollbar-team/release
sudo apt-get update
sudo apt-get upgrade