You found a bug. The bug was: When I was computing the animation frames for the visual, after a backtracking event, I forgot to restore the 'progress' values for the PROGRESS nodes to what they were before the stack push took place. I think I fixed this issue, and I just pushed an update which should appear any second when the cache invalidates.

Cool! Thanks for looking at it/fixing it.

P.S. The whole split() thing you do is an interesting hack to not have to preserve/rewind so much state. :D I've never quite seen fork() used so...