Thanks! This undo function is fun.
The repo at bitbucket.org/StephanNeupert/box-puzzle doesn't yet have the commits, newest commit is from 2017-09-28. I've dabbled in your shipped binary instead.
#6. Undo moves a large box in addition when it should undo a small-box move. To replicate, go to level 3, and perform these moves: up, up, left, right, down, down, left, up. Press undo. Expected: The large blue stays where it is. Observed: The large blue warps two spots downwards, on the pusher. (Guesswork: The undo function prunes too many circles, even when the pusher's circle has moved boxes nontrivially. My incentive for that guess is that, after the above repro instructions, you can press undo once more and are back at the starting position exactly.)
-- Simon