Function paint_focus_ring 

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

The keyboard focus ring around a pressable surface of radius: your colour, 2 px, just outside the surface so it never covers its content. Composites that own their focus (rows, cards) paint it themselves; a painted surface with its own hit-testing calls this.