Nice minimalist implementation of the game mechanic. Fits the theme perfectly.
I do wish the tiles would animate very quickly into their new position rather than popping (to give more visual flow) -- but this would also slightly delay input and could harm "game flow" if not done carefully. It should be *very fast* I would think.
Generally in a Collapse-style puzzle you are not making moves super quickly, so I think the animation should not harm usability as long as it is quick.
Otherwise, very nice casual puzzle!