However, here's one of my ideas.
That's pretty much how it's already set up, though I'm extra certain scroll bars aren't needed for the level display. The levels simply cannot be big enough to warrant the presence of scroll bars. As for the menu bar at the top, I'm still going back and forth on that one. The final product may not have one.
[...] except Lemmix shows it in a separate popup window, and has a vertical rather than horizontal list
I've already begun working on this interface, scroll bar notwithstanding. It can be configured to be vertical or horizontal and on any of the four edges of the screen. The size of the squares is dynamic, too, though I'm not entirely sure I'll let the user change that setting. It can be hidden, though.
While the right-click-and-pan is a reasonably good alternative to a scrollbar, I can see some people might like the visual indication offered by the scrollbar's thumb, allowing a sort of visual-muscular memory to be developed for scrolling to a particular place in order to get a particular preset or object.
Deal. This is a very good point made possible only by the presence of a scroll box. If you can see the relative position in the list of the object you want, you know that it's always "right about there." I'll adjust my layout accordingly.
You appear to be using the operating system's GUI widgets already, so I strongly advise to use the common, known-by-all solution to this design problem.
Thank you.
Why thank you? Because I'm
not using any system UI controls, so the fact that you couldn't tell means I'm doing my job correctly. Everything's handled internally by the program and will look and act exactly the same no matter what system you run it on.