mozilla/pdf.js — pull request #1928 by gigaherz (2012-07-27 13:48:16+00:00)
Title: Improved page tracking on scrolling

I improved the "current page" tracking by using a visibility percent to favour more visible pages instead of using the first page, but keeping the current page if it's still fully visible.

This mimics the behaviour of Adobe Reader if I remember correctly (I don't have it installed in this computer).