clayFocusShadowBackdrops function
The scene colours a key (hence its drop shadow and the focus ring drawn round it) can sit on: the base, the top wash and their mix, sphere B (lit, body, shade and their mixes) and the spheres' shadow core (and its mix with the base). Sphere A stays behind the toolbar, the card or a gutter, never under the keypad.
Implementation
List<int> clayFocusShadowBackdrops(List<int> colors) {
final c = [for (final x in colors) x | 0xFF000000];
return [
c[0],
c[1],
_mixHalf(c[0], c[1]),
c[5],
c[6],
c[7],
_mixHalf(c[5], c[6]),
_mixHalf(c[6], c[7]),
c[8],
_mixHalf(c[8], c[0]),
];
}