Remove slight flickering when going sticky

[bep]

This is in Chrome, but i suspect this is a common issue.

First I would say that this is a great little library that does one thing and one thing good (this is less common than one should think).

But being nit-picky: When the div is going into "sticky mode" there is a slight flickering/jitter-effect that would be nice to smooth out.

Not very important, but nice.