Section nav

Pinned jump links under the header for long pages. The current section takes the yellow underline.

Stipulations

• Sections under it use .sys-section: a thick rule on top, and a scroll margin one rule-width less than the nav height (48px for a 52px nav). A jump then tucks the rule behind the nav instead of leaving a short line under it.
• When a page has a section nav, the header above it scrolls away (position: static) and only the nav stays pinned.
• On phones the links scroll sideways with no visible scroll bar.
• Mark the current link with .is-current from an IntersectionObserver, not a scroll listener.