Function mount_selector 

pub(crate) fn mount_selector(
    world: &mut World,
    ui: Entity,
    switch: Entity,
    current: &'static SceneDef,
    initial: Option<Rect>,
)

Mount the windowed shell’s scene-selector panel.

The panel lists every scene and writes its choice into the SceneSwitch component, which the frame loop reads once after the frame’s advance.