Function draw_child 

pub fn draw_child(
    child: &mut (impl Widget + ?Sized),
    scene: &mut Scene,
    transform: Affine,
    text_renderer: &mut TextRenderer,
    theme: &Theme,
    presence: &PresenceCtx,
)

Draw child unless what it paints misses the viewport. Off-viewport widgets stay dirty, so they re-emit when scrolled back in. A widget ends every layer it begins: the render service refuses a scene that leaves a clip open, and with it the whole frame; debug builds check that here, naming the widget.