Constant FLING_DECAY_PER_SECOND 

pub const FLING_DECAY_PER_SECOND: f64 = 9.0; // 9f64

How quickly a fling slows, per second: velocity decays as v(t) = v₀·e^(−k·t). One rate for every scroll surface, so lists (ScrollMomentum) and documents (Scrollable) glide alike.