Thumbnail view left/right buttons stop working when zoomed in a lot

[cdrini]

To reproduce:

1. Open a book and go to thumbnail view, eg https://archive.org/details/micro_IA40385001_0149/micro_IA40385001_0149/page/18/mode/thumb
2. Zoom in until only two pages are visible
3. Flip left, then flip right

Expected: Something happens
Actual: Nothing happens, it seems to be stuck trying to flip forward, but never actually does.

[deyshift]

It seems to be a problem in any state of zoom that the flip right button will stop working.