Well done! Very clean design and easy to use. Initially, I missed some conveniences of modern piano rolls (like tap and hold to area select a bunch of notes), but ultimately it just made me more careful, which was an interesting change. Some UX decisions were very thoughtful, like the blue vertical lines.

If I could suggest anything, it would be to let fat-fingered users like me hide the left menu in the landscape orientation (mobile) and stretch the roll sideways.