Meanwhile, I checked on this. This touchscreen problem to scroll up and down is definitely a bug. Presently it works only if the zoom setting is 105% or above. It doesn't work for 100% zoom or below.
With any pdf file opened at standard 100% zoom, if it shows 3/4th of the total page length in one view, scroll up or down with touchscreen is not possible to see the balance 1/4th length.
This, combined with the above other bug regarding scrolling issue by button in portrait mode is a handicap. Please check and consider to resolve these issues in the next release of the firmware.
Also why restricting touchscreen scroll up / down feature in case of prc file in fbreader. It is a natural way to scroll the page up by pushing vertical than the present way to doing left or right movement at the bottom. With this feature, user can also use the complete screen area and not only the bottom. Anyway, here it is still only a matter of taste and not a problem.