Z-order not entirely defined

As discovered when fixing #653 ,

- priority sounds different for drawing and mouse events (local-only Z)
- so problems may reveal in nested hierarchy
- raw widgets are of course on top of pbr widgets, nothing particular to do there? But it needs to be told
- what would CSS z-index do when nested?

Start by reviewing issue #653 and the existing drawing and mouse-event handling for nested widgets. Define and document how z-order behaves for drawing, mouse events, raw widgets, and PBR widgets, including the expected nested behavior and any CSS z-index relationship.

Written by the indexing model from the issue text.