Function sideways_travel 

pub fn sideways_travel(
    delta: &MouseScrollDelta,
    scale_factor: f64,
) -> Option<f64>

A wheel event’s sideways travel in logical pixels when horizontal motion dominates, positive for a wheel turned (or fingers scrolling) rightward, which scrolls content toward its later columns; None for vertical motion. Winit reports pixels physical, so they are divided by scale_factor, and a line is a notch of the wheel’s own metric. The desktop’s window strip and the app under the pointer read a wheel by this one rule.