The per-element color/box-shadow transition repainted the whole tree every frame (~5fps). Replaced with document.startViewTransition: the browser cross-fades one GPU-composited snapshot of the page — smooth regardless of element count. flushSync + useLayoutEffect ensure the data-theme flip is captured in the transition; instant fallback where unsupported / reduced-motion.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
ThemeContext adds a .theme-transition class to <html> for ~300ms during a real toggle (skipped on initial mount); a scoped base.css rule applies a uniform 0.3s color/bg/border/fill/stroke/shadow transition to the whole tree only while that class is present — so the entire UI fades together on switch, while normal interactions keep their own transitions. Respects prefers-reduced-motion.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
