fix(docs): reduce vertical scroll jank on docs pages

[quantizor]

Summary

• CaptureScroll: scroll the real .root scrollport instead of window.scrollBy (which did nothing and fought the sidebar wheel handler). Track main-content wheels in capture phase; keep non-passive only where preventDefault is required. Add overscroll-behavior: contain on the sidebar.
• Nested scrollport: leave .root as the scroll container (safe for fixed nav/sidebar), but set scroll-behavior: auto so continuous scrolling stays native. Intentional smooth scrolling stays on docs section clicks via scrollIntoView, with prefers-reduced-motion respected.
• Navbar: remove backdrop-filter: blur(8px) (expensive over scrolling content) and bump navBg alpha 0.85 → 0.94 so the bar still reads solid without frosted-glass cost.
• Scroll spy: rewrite useScrollSpy to IntersectionObserver on .root instead of getBoundingClientRect + setState every scroll frame.

Does not touch live-editor Proxy (#1008) or homepage showcase.

Test plan

• Desktop: open /docs/v7, scroll the main column quickly with the wheel/trackpad — should feel native, no hitching.
• Move cursor onto the sidebar while still scrolling the page — main content should keep scrolling (CaptureScroll redirect).
• Scroll the sidebar alone when idle — sidebar scrolls; at edges, no runaway chaining.
• Click sidebar section links — smooth scroll to section unless prefers-reduced-motion: reduce (then instant).
• Confirm active section highlight still updates while scrolling.
• Mobile: open/close docs nav; sidebar fold + scroll still work.
• Dark/light: navbar still readable without blur.

[vercel]

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Project	Deployment	Actions	Updated
styled-components	Ready	Preview	Sep 11, 2026 4:02am UTC

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Latest commit: c566607

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