Flex-box Footer Element Centered

I'm making a user-resizable GUI window with a header that gains height through new elements, a footer with static height, and a spacer in between that automatically takes up the rest of the height. I attempted using this answer, but the footer ended up vertically-centered. Picture:

If anyone knows why off the top of their head, it would be greatly appreciated. The element is being added to the page with javascript so the code is pretty messy. Thank you for your time.