Only trigger fresh waypoints that are within the viewable area

[brendon]

It doesn’t make sense to trigger fresh waypoints that are already
outside of the visible area of the scrollable context. The waypoints
may be initialised when the context has already been scrolled somewhat.

This change will only trigger a fresh waypoint if that waypoint is
between the top and bottom of the visible area of the scrollable
context. It triggers both forward and back because there is no concept
of just ‘visible’.

[brendon]

Deals with #508. No tests yet as I wasn't able to get the test suite running locally. I wanted to table this change for discussion as it changes previous behaviour that may have been unintentional but still relied upon.

[brendon]

@imakewebthings, were you interested in looking into this change further?

[dzajic]

I ran into this, in a painful way. I think it's totally surprising for it to behave the way it does. I tried your fix and it seems to work. The last commit was over a year ago (9/2016), and there over a thousand forks, so I'm guessing this is 💀. Thanks for the fix, @brendon!

[brendon]

Thanks @dzajic. It's a great project. @imakewebthings Caleb, would you be interested in bringing on some extra maintainers? I hate to see good software die! :)