Function shape_group_fn 

pub fn shape_group_fn(
    shapes: impl Fn(Size) -> Vec<ShapePiece> + 'static,
) -> AnyPiece

Size-aware shape_group: the closure derives the shapes from the laid-out size and re-runs on FrameChanged, exactly like canvas — for geometry that depends on the final size (e.g. data mapped along the width).