Function decide 

pub fn decide(
    before: Option<f64>,
    after: Option<f64>,
    steps: i32,
    step: &Step,
) -> Decision

Decide what the measurement means.

before and after are the scroll-bar values around the wheel event, or None when the bar could not be read. step is the sized per-line fraction; the caller passes the same Step from the read pass, so the AxScrollbar decision uses the same math that produced the warning. The order matters: a bar that moved wins over a bar that sits at its limit, because a bar sitting at its limit cannot have moved.