Function panel_shadow 

pub fn panel_shadow(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    radius: f64,
    theme: &Theme,
    intensity: f32,
)

Lay down a floating panel’s elevation, clipped to the ring outside its own silhouette. The shadow is crate::surface::shadow, a blur of the panel’s own rounded rect (so it stays even around a corner instead of showing a flat-edged ghost of a square one). The ring clip stays even though chrome is opaque: a blurred rect is solid in the middle, and a half-faded panel (intensity) must not darken its own interior on the way in.