`onLeave` isn't fired when `onPositionChange` does

        <Waypoint
          onPositionChange={position => console.log(position)}
          scrollableAncestor={typeof window !== 'undefined' && window}
          onEnter={this.showHeaderButtons}
          onLeave={this.hideHeaderButtons} />

onLeave fires when scrolling normally. But when scrolled to the bottom of the page and then refreshing the page the browser scrolls to the bottom when the page has been loaded and while onPositionChange fires correctly with the above updated position onLeave doesn’t get called at all.

[jamesplease]

As an aside, I could be convinced of there being a callback as a prop that is fired immediately when the component mounts, which is passed the initial position. To implement this, we would need the callback prop to be called after this line.

An example API would be:

<Waypoint
  onMount={initialPosition => console.log(`This waypoint mounted at ${initialPosition}`)}
  onEnter={this.onEnter}/>

I’m not convinced that we need this right now, but if someone could show that it:

1. enables them to do something they cannot do with a workaround
2. enables them to clean up a really messy workaround to be much cleaner

then I’d likely be 👍 for this new prop.

[jamesplease]

I was thinking that, @lencioni . I can PR a thing up later today, unless you’d like to take a stab at it.