Function behind_panel 

pub fn behind_panel(
    scene: &mut Scene,
    transform: Affine,
    window: Rect,
    panel: RoundedRect,
    theme: &Theme,
)

Dim the whole window behind a modal Panel, the part under the panel included. Floating material blurs the scene recorded behind it, so the scrim belongs under the material: a bright control behind the panel (a primary button) is dimmed with everything else before it is blurred, instead of glowing through the sheet as a coloured smudge. panel is where the sheet stands; the dim does not stop at its edge.