Function paint_app_owned_content 

pub fn paint_app_owned_content(
    scene: &mut Scene,
    transform: Affine,
    content_rect: Rect,
    theme: &Theme,
    draw: impl FnOnce(&mut Scene) -> bool,
) -> bool

Paint the framework-owned frame around one interactive app-owned surface.

Retained app content stands on the ordinary opaque Object ground, not a translucent Panel material. The shared floating silhouette clips every app paint, and its rim is drawn last so square app backgrounds cannot erase the frame. The native allocation remains transparent outside the silhouette and its shadow; hosts allocate it with app_owned_content_window_rect.