Function floating_edge 

pub fn floating_edge(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    radius: f64,
    theme: &Theme,
)

The 1 px inner hairline a floating surface carries on its outermost pixel row (crate::chrome::panel_border): white at 7 % on a dark material, black at 8 % on a light one, full strength on high contrast. Paint it after the surface’s body, at the surface’s own radius.