This is a good idea, and something that I think would be interesting beyond seeing just how people solved katas, but I think that it would be much much better if you had the transition between commits happen in place instead of sliding in. The sliding transition makes it so much harder to see what changed, whereas if you just had the visible code change in place instantly, people can jump back and forth between two commits and easily see what has changed. Maybe even give the new code some highlights or something to make it stand out.

Plus a slider to see where you are in the timeline would be sweet.

Great idea!