Comment 4 for bug 1261600

Indeed, it looks fixed here too after updating. I suspect the cause might have been that the underlying gtk-3.0 library was upgraded to version 3.10 while gnome-sudoku was still at 3.8. Might have been some mismatch or change between the two versions causing the strange behaviour which got sorted out once gnome-sudoku was upgraded to 3.10 as well.

Since sudoku now works again, I'm marking this Fix released.