Feature: children can also be a function returning the actual children

[jtomaszewski]

This lets the children prop to be a function, which will receive { absolute: bool, sticky: bool, disabled: bool } object as a first argument and should return a child or children.

Example usage: <Sticky>{({ sticky }) => <Menu isCollapsed={sticky} />}</Sticky>

This is useful if your component is dependant on whether the element is currently being sticky or not, and depending on the css class is not enough for you.