Function edge_on 

pub fn edge_on(surface: AlphaColor<Srgb>) -> AlphaColor<Srgb>

The derived 1 px rest edge of a control, card or row: a hairline measured against the surface it is painted on, never a declared border_standard (which is one fixed colour per theme and disappears on a raised dark surface). Walks EDGE_ALPHA_STEPS until the edge visibly registers (tokens::colors::HAIRLINE_MIN_DELTA).