Function overlay_elements 

pub fn overlay_elements<R>(
    wm: Option<&WmState>,
    renderer: &mut R,
    origin: Point<i32, Logical>,
    size: Size<i32, Logical>,
    scale: f64,
) -> Vec<MemoryRenderBufferRenderElement<R>>
where
    R: Renderer + ImportMem,
    R::TextureId: Texture + Clone + Send + 'static,

The render elements of every overlay that touches an output at origin of size (logical), topmost first.