Try to connect the not apple's mouse (USB or bluetooth), disconnect the apple's mouse and you can see the problem, the scroll appear without the background color assigned. In all applications.

No one saves common data to disk?

OK the problem was alphaValue, I don't know how Apple handles it with an external not Apple´s mouse, but override in NSScroller the mouseExited, mouseEntered and set alphaValue to 1.0 all fixed