Hmm. I just realised this problem only occurs when I'm viewing a document in 'two-page' view, where two pages are viewed side-by-side. Is this a bug? Do other people have this same issue?

EDIT: Found some discussion on this issue here. Though no true solution was offered.

My 'workaround' will simply be to use single-page viewing. However, I'd like this scrolling capability while in two-page view. I can't imagine why it would be disabled for that mode, other than it being a bug. The main reason I like to be able to scroll as I've described is so that I can momentarily scroll down to a point in the document and look at something, and then move my cursor off the slider toward the middle of the screen (while still holding down click) - which causes the current viewing position to return to the initial position when the scroll-bar was first clicked and held - meaning I don't lose my place.