The frame an application mounts once and routes into. children defaults to <NestedView />, so a real app passes nothing here and lets the router fill it. The rail is live: every entry is a link back to this page with a different ?nav=, which is what moves the active marker.

• sidebar - flush, side by side.
• floating - the page owns the background, the rail is a card on it.
• inset - the rail owns the background, the page is the card.
• headerOutside applies to inset alone, so its switch does nothing on the other two.

The rail itself, its groups, badges and sub-menus, has its own page at /blocks/sidebar. This one is about the frame around it: the top bar, the breadcrumbs, and how the two surfaces meet.