The click and drag controls to select free-ness could be improved by only altering cells sharing a class with the original. This prevents the situation of clicking on a green cell, dragging to another green and accidentally changing the classes of red and gray cells on the way.
It may also be nice to allow for another drag mode that bypasses the current 'daisy-chain' toggle of gray -> green -> red, perhaps by also pressing shift. This mode would set all cells the mouse enters to the same class as the starting one.
The click and drag controls to select free-ness could be improved by only altering cells sharing a class with the original. This prevents the situation of clicking on a green cell, dragging to another green and accidentally changing the classes of red and gray cells on the way.
It may also be nice to allow for another drag mode that bypasses the current 'daisy-chain' toggle of gray -> green -> red, perhaps by also pressing shift. This mode would set all cells the mouse enters to the same class as the starting one.