Function push_layer_elements 

pub(crate) fn push_layer_elements<R>(
    renderer: &mut R,
    elements: &mut Vec<CompositionElements<R, WaylandSurfaceRenderElement<R>>>,
    layer_map: &LayerMap,
    target_layer: Layer,
    scale: f64,
)
where
    R: Renderer + ImportAll + ImportMem,
    R::TextureId: Clone + 'static,

Push an output’s wlr-layer surfaces on target_layer into elements, top-most first, the order layer_map.layers() reversed gives, each with its popups and subsurfaces. Shared by both renderers and by both of a frame’s layer passes, which differ only in the layers they name.