There are two verisons of each element. One is hidden & fixed, and the other is visible & absolute.
Both are positioned with respect to the viewport.
Fixed objects ~ "display: none"
When a container scrolls 100% into view the fixed element is shown, and the block element is hidden. Since containers 2 & 3 are inside one, elements only revert when you scroll back up!
Hope this makes sense. If not, please let me know in the comments.