Normally, fixed position elements are positioned at `top: 0`.
The code that stops them from overlapping the footer does so by:
- setting the position to absolulte
- setting `top` to a calculated value (eg `1500px`) which makes it sit
above the footer
The problem is caused because we’re animating `top`, so when scrolling
back up the page elements are getting animated from `1500px` to `0px`,
ie flying up from the bottom of the screen.