Function draw_child_at 

pub fn draw_child_at(
    child: &mut (impl Widget + ?Sized),
    index: Option<usize>,
    scene: &mut Scene,
    transform: Affine,
    text_renderer: &mut TextRenderer,
    theme: &Theme,
    presence: &PresenceCtx,
)

draw_child for a container that knows child’s place among its children (index), so a host that keys widgets (crate::scene_parts::key_widgets) keys it by that place even when the container leaves siblings before it undrawn.