hakimel/reveal.js — pull request #778 by ccortezia (2014-01-16 16:50:51+00:00)
Title: Fixes bad NaN applied to style on updateParallax.

The previous criteria "verticalSlideCount > 0" would result
in verticalOffset being NaN when verticalSlideCount == 1.

This would cause dom.background.style.backgroundPosition to
be set to something like "123px NaNpx", ultimately preventing
the parallax effect to play (silently failing so far).