Function composition_elements 

pub(crate) fn composition_elements<R>(
    renderer: &mut R,
    scene: &FrameScene<'_>,
) -> Vec<CompositionElements<R, WaylandSurfaceRenderElement<R>>>
where
    R: Renderer + ImportAll + ImportMem,
    R::TextureId: Clone + Send + 'static,

The render elements of an output’s frame, topmost first: the software cursor, a drag’s icon, the watermark, the window manager’s overlays, the overlay and top wlr-layer layers, the windows with their frames, and the bottom and background layers. One list for both renderers, read by scanout_candidate before anything is drawn.