ConstA surface stacks its built-in layers, then the element's layers, then the registered layers in
registration order, then those declared with SheetLayers. A layer's zIndex overrides that
order. A registered layer is mounted on every sheet it matches, on each pane, so it renders an empty
Layer while it has nothing to show.
The layers plugins add to sheets. Every sheet surface, in every workbook or sheet element, renders the registered layers that match it: