Animate fixed elements on scroll events

Utilize scroll events and scroll offset to show/hide fixed elements. This would allow designers to make an element sticky as a user scrolls past a certain x or y point. This is a common pattern for bottom bar menus, submenus, and section headers, especially on mobile. It would also be helpful to trigger an animation when a user switches scroll direction, i.e., show a fixed header only when a user scrolls up.