Function card_on 

pub fn card_on(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    radius: f64,
    fill: AlphaColor<Srgb>,
    ground: AlphaColor<Srgb>,
)

card for a translucent fill (an accent wash, a hover tint): the edge derives from what the eye sees — fill composited over ground — not from the wash’s own RGB. A 16 % accent wash on a light panel is still a light surface and wants a dark hairline; judged by the accent alone it would get a white one and vanish.