scrollStep property
Base distance, in logical pixels, that one mouse-wheel notch scrolls the
time axis at zoom 1. The effective step is scaled by the current zoom
(Controller.verticalScroll) so a single notch always moves the same
amount of time regardless of zoom (the old code used a fixed 20px step
that moved less time the further you zoomed in). Defaults to 20.
Implementation
double scrollStep;