Function bind_surfaces 

pub fn bind_surfaces(
    commands: Commands<'_, '_>,
    surfaces: ResMut<'_, Surfaces>,
    roots: Query<'_, '_, (Entity, &RSurface, Option<&UiTargetCamera>, &mut Visibility)>,
)

Spawn a UI camera for each registered surface, then bind every <surface> root to its camera (and hide roots whose name isn’t registered, so they never spill onto the main screen). Runs after the reconciler op drain so a freshly-mounted surface binds the same frame.