Function velocity_past_press 

pub fn velocity_past_press(
    pointer: f64,
    press: f64,
    start: f64,
    end: f64,
    band: f64,
) -> f64

velocity for a drag that was pressed at press, over a band band deep: it scrolls toward an edge only once the pointer has gone further that way than where it was pressed. A press already inside the band does not scroll by itself, and a drag back inward never scrolls outward.