Highlight invalid pencil marks in cells

[Naomitr0n]

Currently if you fill in a square with a number, any pencil marks for that number in the same column or row are removed. An option to prevent pencil marks from being made if the number you're trying to mark already exists in the same column or row would be great.

In other words: if you have a filled 3, you can currently put a pencil mark of 3 in any unfilled square directly next to it. This option would prevent that.

For visual feedback, maybe have the number(s) that make it invalid blink or some such? I'd be weary of changing keypad coloring as that would effectively show a "per-square" pencil mark list and make it very obvious when there's only one option available, which isn't the goal of this ticket.

I say "option" because I can see some use cases where a player might not want this to be forced behavior. For example, if the player realizes they have a square filled wrong and are using pencil marks to figure out where they messed up. Probably far more obviously though is that sometimes finding out your own mistake in the pencil marks can be a challenge in and of itself and adds to the difficulty of harder levels.