Scroll Motion

scroll-motion

Add motion that runs on the compositor thread, points toward the action, and degrades gracefully — not a heavy JS library for a fade.

The failure this fixes

Asked to "add scroll animation", a model reaches for the heaviest tool it knows — Framer Motion for a decorative fade, Three.js for a hero — when a few lines of native CSS would run for free on the compositor thread. It then animates layout-triggering properties (width, height, top, box-shadow), forcing every frame onto the main thread, which competes with interaction handling and tanks INP and CLS on the mobile devices that are 60%+ of traffic. The second half of the scar is narrative: it scroll-jacks and gates content behind a scroll "tunnel" with no exit to pricing/CTA — the most-hated anti-pattern (NN/g) and an accessibility failure.

When to use / when NOT to use

Use for any scroll/motion decision: reveals, parallax, progress bars, pinned/scrubbed sequences, scrollytelling, smooth-scroll, and choosing between CSS / GSAP / Motion.

Not for: the words (landing-page-copywriting), section order (landing-page-structure), palette/type (visual-design-system), general performance/SEO (web-vitals-and-seo — this skill owns motion's INP impact specifically), or full WCAG work (landing-page-accessibility — this skill owns the motion accessibility floor).

Workflow

1. Classify the animation by purpose, then pick the tier (climb from the top; drop down only when the tier above structurally can't do it). CSS covers ~80% of what ScrollTrigger used to be needed for.

   Need	Reach for	INP impact
   Decorative reveals, fade/slide-in, progress bar, simple parallax, staggered grids	Native CSS animation-timeline: scroll() / view()	None (compositor thread)
   Pinned sections, scrubbed timelines, horizontal-in-vertical, SVG path	GSAP + ScrollTrigger (via @gsap/react useGSAP())	Low if optimized
   React component state, gestures, layout animation, AnimatePresence exits	Motion (formerly Framer Motion)	Higher; use LazyMotion
   Immersive 3D scroll scenes (brand/premium only)	react-three-fiber, sparingly	High; prefer pre-rendered sequences

   Hybrid stacking all tiers on one page is the normal professional pattern.

2. Keep it compositor-safe. Animate only transform, opacity, filter. Compositor eligibility is all-or-nothing per @keyframes block — one layout-triggering property drops the whole animation to the main thread.

3. Handle the CSS gotchas. animation-fill-mode: both is mandatory (or reveals flicker back to their start state); scroll() tracks container progress, view() tracks one element through the viewport; wrap in @supports (animation-timeline: scroll()) with a static, fully-visible fallback (Firefox lacks it). Details: references/motion-stack.md.

4. Gate motion for accessibility. prefers-reduced-motion is the floor, not sufficient — reduce (swap displacement for an opacity cross-fade), don't delete; reveal content must be structurally present regardless of animation; auto-play >5s needs a pause control (WCAG 2.2.2). Full rules: references/motion-stack.md.

5. Never gate content. Motion points toward the action; keep visible exit points and CTAs; never scroll-jack. Cap scrollytelling at ~5 scenes / 2-3 min, and apply the litmus test: if removing the scroll-link still reads fine as a static page, it was decoration, not scrollytelling — write the narrative in prose first.

The rules

• Purpose picks the tier, not what's already imported. A simple reveal in Framer Motion is the canonical waste.
• Only transform / opacity / filter animate cheaply; everything else triggers layout/paint every frame.
• animation-fill-mode: both on every scroll reveal.
• @supports fallback because Firefox lacks animation-timeline; unsupported browsers render the final visible state, never a hidden one.
• Lenis smooth-scroll breaks position: sticky, IntersectionObserver, and CSS scroll-driven animations unless run in native mode (ReactLenis root). Only add it to coordinate GSAP + Motion under one rAF loop.
• GSAP is free for commercial use (Webflow/GreenSock, since April 2025), including ScrollTrigger and SplitText — it is not MIT-licensed; it uses the GreenSock standard license. useGSAP() auto-cleans tweens on unmount.
• In Next.js/RSC, JS animation libraries force a contagious "use client" boundary — push it to the leaf; CSS scroll-driven animation forces none (another reason to default to it). Boundary mechanics live in nextjs-landing-page.

Output

Motion implemented at the lowest-cost tier that fits the purpose, animating only compositor-safe properties, with @supports fallbacks and reduced-motion gating, no scroll-jacking, and any browser-support claim version-gated against caniuse.

References

• references/motion-stack.md - the full tiered decision, CSS gotchas, the accessibility floor, per-library reduced-motion hooks, GSAP pin pitfalls, and the fact ledger (stable vs drift-prone vs unverifiable).