Regarding your suggestion, I did think about that, but didn't do it because then you could easily manipulate the shapes i.e. construct border with other pieces so that desired piece expands desirably and then removing the other pieces.
I thought that if you pick up a piece from board it could recalculate all others. So it would not break main idea, that each piece needs to expand fully. But yes, you are right it would be much easier, and requires more logic and additional visual indication.
