Comment by iamben
This is great fun, congrats!
2 small requests that (I think) would help with the UX: consider moving (or duplicating) the play button - maybe directly in the middle below the editor, or on that panel itself. It took me a few confusing seconds to realise where it was. Also, could you consider making each fourth (or first) column a very slightly lighter grey? So if I want my kick on 1, 3, 10 and 11 it's really easy to see where to click without counting?
Great ideas, thank you. I will triage these suggestions into the issue tracker. Many thanks!