Missing repainting in Tree in Ensemble demo when moving mouse

I doubleclick the Ensemble.jar demo in such a way that the mouse pointer will be on top
of the word "Interpolator". After doubleclicking I don't move the mouse anymore until the
window appears. The Interpolator example is highlighted. Now I move the mouse pointer
to the left into the tree that lists all examples. In the free grey/blueish area I move the
mouse up and down, and this creates these artifact rectangles as you can see in the
attached screenshots.