Toggle for "cheats"

[Snowflake6]

It looks like incorrect letters are left as the sketched letter, and the interpreted letter is shown in the bottom corner of each box as it's drawn. Purists don't really want to know if the letter is "correct" as they do the crossword, that's half the challenge... It's common to have a wrong word in a slot right up until you get the crossing words conflicting with it and you have to resolve it. Getting things "confirmed" as you go is a "cheat."

So maybe a three-way toggle to select no checking to be done at all, letter-by-letter checking as you go, or a crossword-wide check when you're finished?

[Snowflake6]

Further to this... My wife does a lot more crosswords than I do, and in addition to the feedback above, says that when the cells go all-black, it makes it very hard to distinguish between the word-space and the walls. She too would prefer to not see whether her entry is correct, but if it's going to change colour, maybe it could go to some lighter grey than the "wall" cells, so you can still see how long the "word" space is.