There are following problems:

ScrollPane:
    scrolling above scrollbar behaves differently from scrolling above the content

ScrollBar:
    horizontal scroll events move vertical scrollbar
    currently it moves the scrollbar about the number of pixels reported by the event, which is wrong, the values are meant for content. This way it can scroll many pages per each mouse wheel notch.

VirtualFlow:
    scrolling above scrollbar behaves differently from scrolling above the content
    line scrolling is weird. It uses textDeltaX without looking at its units. The units will probably be "characters", but it uses the value as "cells", which is something totally different (many times bigger). Also interpreting "3 lines" for horizontal scrolling doesn't make much sense.
    horizontal scroll events move vertical scrollbar