Graphical error in scroll thumb in Ensemble demo

I run the Ensemble demo and move my mouse pointer over one of the little Icons, for example over "Interpolator" so that the animation effect of that icon is visible for at least a short amount of time. Then I move my mouse pointer on the thumb of the scroll bar and at this moment a graphical error appears. As soon I move the thumb it looks okay again. But moving the mouse pointer again to the lower end of the thumb reproduces this effect for me. Please see the attached screenshot.