There are many ways to scroll the list in WindowBlinds 7.
1) The keyboard up / down / left / right arrows
2) The keyboard page up / down buttons
3) A wheel mouse
4) Clicking & dragging on the bottom 15 or so pixels in line with the bar that appears. You do NOT have to select the bar itself so the fact it becomes smaller with more skins isn't an issue. The only thing that matters is the vertical position.
5) Clicking and dragging on the area thats above the stars in the list. This drags the list and if you do quick throws of it the list will scroll.
Why is the scrollbar not showing until mouseover a problem? You can't scroll it using the mouse unless you are over the control anyway and hiding it away keeps the UI clean.