position: sticky is a new way to position elements and is conceptually similar to position: fixed. The difference is that a stickily positioned element behaves like position: relative within its parent, until a given offset threshold is met.
Docs: https://developer.mozilla.org/en-US/docs/Web/CSS/position#Sticky_positioning
Samples: https://ebidel.github.io/demos/sticky-position-event.html