auouymous, you are right, but this will make it impossible to do transparent drawing on the top of the scrolled area. That's exactly what I meant here:
If we will blit already rendered window contents, this will 'scroll' transparent elements too. But if we introduce the limitation that no transparent elements can be drawn over scrolling areas, then yes, it is possibly can be improved as you described.
In the current Kroll's design this will affect round corners around the playlist and seekbar's glowing.