Something not working, or an idea for Stackpick? Write to javier.bascones@gmail.com — you'll get an answer from the person who made it, usually within a couple of days.
Not yet: UNDO works even after the tray fills up. Take back a move or two and try another order.
It asks a solver for a move that still leads to an empty box, and highlights it. If there is none, it tells you to undo.
No. The game has no networking code at all.
No. It is a universal purchase: buy once and it works on iPhone, iPad and Mac with the same Apple Account.
iOS 15 or later, or macOS 11 or later. The game runs entirely offline.
Stackpick