Function edge_delta 

pub fn edge_delta(edge: AlphaColor<Srgb>, surface: AlphaColor<Srgb>) -> f64

How far edge moves surface once painted on it, in relative-luminance units — the measurement HIG §9.5 asks every chrome border to pass. Uses the same compositor as Colors::hairline_on / lift_target / window_edge, so one rounding model decides “visible” everywhere.