Function hinted 

pub fn hinted<R>(
    scene: &mut Scene,
    key: PartKey,
    hint: Option<DamageHint>,
    draw: impl FnOnce(&mut Scene) -> R,
) -> R

Draw one part of a section with no bounds of its own: draw paints content whose change since an earlier frame hint describes. A change inside the part damages the hint’s rects when the hint follows on from the compared frame’s (same source and placement, the next generation). Otherwise, or with no hint, the change belongs to the enclosing part, and outside every bounded part it is not proven.