fix(site): scroll-spy highlights the last guide section at page bottom#207
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The "On this page" rail used an IntersectionObserver with a narrow `-20% 0px -65% 0px` band: a heading only went active while inside the top 20–35% of the viewport. Headings near the page bottom never scroll high enough to enter that band, so the final section(s) never activated — at the very bottom the rail stayed stuck on whichever heading was last in the band (often the first). Replace it with a scroll-position computation: the active heading is the last one whose top has crossed a line ~28% down the viewport, and the last heading is forced once scrolled to the page bottom. rAF-throttled scroll/resize listeners. Verified via Playwright at top / middle / bottom — correct heading active in each, exactly one active at a time.
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The "On this page" rail used an
IntersectionObserverwith a narrow-20% 0px -65% 0pxband — a heading only went active while inside the top 20–35% of the viewport. Headings near the page bottom never scroll high enough to enter that band, so the final section(s) never activated; at the very bottom the rail stayed stuck (often on the first heading).Replaced with a scroll-position computation: active = the last heading whose top has crossed a line ~28% down the viewport, and the last heading is forced once scrolled to the page bottom. rAF-throttled scroll/resize.
Verified via Playwright at top / middle / bottom — correct heading active in each, exactly one active at a time. (Repro before fix: at bottom active was "Scene-scoped bindings" instead of "Where to go next".)