Currently, our logic for page panning isn't great. If the user starts a
pan upwards and hesitates over a new page, we'll go to the *next* page
on release, since the difference is greater, but the velocity wound down
to 0.
Instead of trying to treat it like page down or scrolls, simply do the
math to find the page where the user scrolled to.
This is unfortunately broken for fast swipes, since the user doesn't get
far enough into the new page to make a difference. I'm getting the
impression we'll need a gesture recognizer for this, though, however
crude. Simple hacks I tried, like a velocity multiplier, didn't work
properly.
https://bugzilla.gnome.org/show_bug.cgi?id=729064
28c1f81f4a