Filling Selection makes color go outside of selection

Hi, I need to fill in selections without a single pixle going outside of the selection, aka without without any amount of color leak.

Here's an example of what I mean. I know it's a very small amount, but you can see a tiny blur outside of the green, going onto the blue.

I have anti-alaising off, and I have Feather set to 0. Yet I will still get a small amount of smoothing, or going outside of the selection. I'm making a mask for another program, so I really need it to be solid only. How do I prevent this?

Thank you so much for any help!