amp-story-desktop-one-panel System layer positioning

[processprocess]

Description

Position the system layer so they are overlaid on story.
CSS code will be repeated from amp-story-page positioning.

Alternatives Considered

We considered a wrapper element for the pages and system layer but that would require reparenting all pages and runtime. This would be a performance hit and would require a re-measure of essentially the entire story.

It also effects all other UI types by adding an additional wrapper, increasing surface for bugs.

Additional Context

No response