ScrollBar increase()/decrease() don't align with mouse snap positions

I'm trying to implement discrete scrolling where the step size equals the height of an item in a ListView.

I can calculate the ScrollBar::stepSize so that the snap points (when dragged) align perfectly with the beginning of the next item. The problem is, that using the ScrollBar's increase or decrease methods will then jump too far. The ScrollBar calculates snapPositions based on the given step size which will always result in a smaller grid then using just increase/decrease which will just add or subtract the step size as is.

This makes it impossible to achieve a great UX, because one of the two use cases will not work.