next page button sometimes doesn't work in zoomed mode

[GoogleCodeExporter]

Your model / PRS+ version?

PRS-505 / latest 2.1.1alpha

What steps will reproduce the problem?
1. open pdf or fb2
2. set larger font
3. press prev and next page several times somewhere in the middle

What is the expected output? What do you see instead?

Workable next button, but it stops working.
Jumping to next page by number works. But not sure if it works for all pages. 
IIRR I jumped to -2 or +2, because jusm to number+1 doesn't worked too.

Original issue reported on code.google.com by koct9i on 27 Jul 2014 at 9:45