Function fill_role_flush 

pub fn fill_role_flush(
    scene: &mut Scene,
    transform: Affine,
    shape: &RoundedRect,
    role: SurfaceRole,
    finish: &Finish,
    theme: &Theme,
    ground: AlphaColor<Srgb>,
    flush: Sides,
)

fill_role for a surface whose flush sides stand on the display’s own edge: it draws no edge, rim or hairline there, so no pixel of another colour shows between it and the screen’s edge. Every other side keeps what its material gives it.