Button height not going to 0%. What's going on?

I’m practicing designing a button interaction via a Youtube video many might be familiar with here. I get to a point where I am supposed to adjust the height of the “hovercolor” class, and it’s supposed to look like a thin line (you shouldn’t be able to see any blue- just a line. But when I adjust the height to 0%, it doesn’t go all the way down. It keeps turning into an oval shape. What am I doing wrong here?

Read-only link