SmoothScroll

npx motionfoundry add smooth-scroll

1import { SmoothScroll } from "@motionfoundry/ui";2 3export function Example() {4  return (5    <SmoothScroll />6  );7}

1"use client";2 3import * as React from "react";4import styles from "../styles/smooth-scroll.module.css";5 6interface SmoothScrollState {7  /** Current scrollTop of the viewport in px */8  scroll: number;9  /** Maximum scrollTop in px */10  limit: number;11  /** scroll / limit, 0..1 */12  progress: number;13  /** Signed scroll velocity in px per 60fps frame */14  velocity: number;15}16 17interface SmoothScrollOptions {18  lerp: number;19  wheelMultiplier: number;20  smooth: boolean;21  onScroll?: (state: SmoothScrollState) => void;22}23 24const clamp = (v: number, min: number, max: number) => Math.min(max, Math.max(min, v));25 26function usePrefersReducedMotion(): boolean {27  const [reduced, setReduced] = React.useState(false);28  React.useEffect(() => {29    const mq = window.matchMedia("(prefers-reduced-motion: reduce)");30    const update = () => setReduced(mq.matches);31    update();32    mq.addEventListener("change", update);33    return () => mq.removeEventListener("change", update);34  }, []);35  return reduced;36}37 38/** True when a nested scrollable element under the pointer can still scroll in the wheel direction. */39function nestedCanScroll(node: EventTarget | null, root: HTMLElement, dy: number): boolean {40  let el: Element | null = node instanceof Element ? node : null;41  while (el && el !== root) {42    if (el instanceof HTMLElement) {43      if (el.dataset.mfScrollPrevent !== undefined) return true;44      const overflowY = window.getComputedStyle(el).overflowY;45      if ((overflowY === "auto" || overflowY === "scroll") && el.scrollHeight > el.clientHeight + 1) {46        const canMove = dy < 0 ? el.scrollTop > 0 : el.scrollTop + el.clientHeight < el.scrollHeight - 1;47        if (canMove) return true;48      }49    }50    el = el.parentElement;51  }52  return false;53}54 55/**56 * Self-contained smooth scroller for an overflow container. Wheel input updates a target57 * position; a rAF loop eases the real scrollTop toward it (frame-rate independent lerp) and58 * stops once settled. Touch, keyboard and scrollbar drags stay native. At the top/bottom edge59 * wheel events are released so the page keeps scrolling.60 */61function useSmoothScroll(62  containerRef: React.RefObject<HTMLElement | null>,63  { lerp, wheelMultiplier, smooth, onScroll }: SmoothScrollOptions,64): React.RefObject<SmoothScrollState> {65  const stateRef = React.useRef<SmoothScrollState>({ scroll: 0, limit: 0, progress: 0, velocity: 0 });66  const callbackRef = React.useRef(onScroll);67  React.useEffect(() => {68    callbackRef.current = onScroll;69  }, [onScroll]);70 71  React.useEffect(() => {72    const el = containerRef.current;73    if (!el) return;74    const ease = clamp(Number.isFinite(lerp) ? lerp : 0.1, 0.01, 1);75    const multiplier = Number.isFinite(wheelMultiplier) ? wheelMultiplier : 1;76    let target = el.scrollTop;77    let current = target;78    let raf = 0;79    let lastTime = 0;80    let lastNative = el.scrollTop;81    let lastNativeTime = 0;82    let settleTimer: ReturnType<typeof setTimeout> | undefined;83 84    const getLimit = () => Math.max(0, el.scrollHeight - el.clientHeight);85 86    const emit = (velocity: number) => {87      const limit = getLimit();88      const scroll = clamp(el.scrollTop, 0, limit);89      lastNative = el.scrollTop;90      const s = stateRef.current;91      s.scroll = scroll;92      s.limit = limit;93      s.progress = limit > 0 ? scroll / limit : 0;94      s.velocity = velocity;95      callbackRef.current?.(s);96    };97 98    const tick = (time: number) => {99      const dt = lastTime ? Math.min(64, Math.max(1, time - lastTime)) : 16.667;100      lastTime = time;101      const limit = getLimit();102      target = clamp(target, 0, limit);103      if (Math.abs(el.scrollTop - current) > 2) {104        // Something else moved the viewport (scrollbar drag, keyboard, touch): hand control back.105        current = target = el.scrollTop;106        raf = 0;107        lastTime = 0;108        emit(0);109        return;110      }111      const prev = current;112      const frames = dt / 16.667;113      current += (target - current) * (1 - Math.pow(1 - ease, frames));114      if (Math.abs(target - current) < 0.35) current = target;115      el.scrollTop = current;116      emit((current - prev) / frames);117      if (current === target) {118        raf = 0;119        lastTime = 0;120        emit(0);121        return;122      }123      raf = requestAnimationFrame(tick);124    };125 126    const onWheel = (event: WheelEvent) => {127      if (event.ctrlKey || event.defaultPrevented) return;128      if (Math.abs(event.deltaX) > Math.abs(event.deltaY)) return;129      let dy = event.deltaY;130      if (event.deltaMode === 1) dy *= 16;131      else if (event.deltaMode === 2) dy *= el.clientHeight;132      dy *= multiplier;133      if (dy === 0 || nestedCanScroll(event.target, el, dy)) return;134      const limit = getLimit();135      if (!raf) current = target = el.scrollTop;136      // Release to native scrolling at the edges so the page is never trapped.137      if ((dy < 0 && target <= 0) || (dy > 0 && target >= limit - 0.5)) return;138      event.preventDefault();139      target = clamp(target + dy, 0, limit);140      if (!raf) {141        lastTime = 0;142        raf = requestAnimationFrame(tick);143      }144    };145 146    const onNativeScroll = () => {147      if (raf) return;148      const top = el.scrollTop;149      const delta = top - lastNative;150      if (delta === 0) return;151      const now = performance.now();152      const dt = lastNativeTime ? clamp(now - lastNativeTime, 8, 100) : 16.667;153      lastNativeTime = now;154      current = target = top;155      emit(delta / (dt / 16.667));156      if (settleTimer) clearTimeout(settleTimer);157      settleTimer = setTimeout(() => {158        lastNativeTime = 0;159        emit(0);160      }, 90);161    };162 163    if (smooth) el.addEventListener("wheel", onWheel, { passive: false });164    el.addEventListener("scroll", onNativeScroll, { passive: true });165    const ro = new ResizeObserver(() => emit(stateRef.current.velocity));166    ro.observe(el);167    if (el.firstElementChild) ro.observe(el.firstElementChild);168    emit(0);169 170    return () => {171      if (smooth) el.removeEventListener("wheel", onWheel);172      el.removeEventListener("scroll", onNativeScroll);173      ro.disconnect();174      if (raf) cancelAnimationFrame(raf);175      if (settleTimer) clearTimeout(settleTimer);176    };177  }, [containerRef, lerp, wheelMultiplier, smooth]);178 179  return stateRef;180}181 182export interface SmoothScrollProps {183  /** Scrollable content (defaults to a set of editorial demo sections) */184  children?: React.ReactNode;185  /** Viewport height in px */186  height?: number;187  /** Viewport corner radius in px */188  borderRadius?: number;189  /** Interpolation factor per 60fps frame (0.05 = floaty, 0.3 = snappy) */190  lerp?: number;191  /** Multiplier applied to wheel / trackpad deltas */192  wheelMultiplier?: number;193  /** Enable smooth wheel scrolling (native scrolling when false or under reduced motion) */194  smooth?: boolean;195  /** Show a scroll progress bar along the top edge */196  showProgress?: boolean;197  /** Progress bar and accent color */198  progressColor?: string;199  /** Show a live velocity readout badge */200  showVelocity?: boolean;201  /** Accessible label for the scroll region */202  ariaLabel?: string;203  /** Called on every scroll frame with scroll, limit, progress (0..1) and velocity (px/frame) */204  onScroll?: (state: { scroll: number; limit: number; progress: number; velocity: number }) => void;205  /** Optional className merged onto the root */206  className?: string;207  /** Inline styles merged onto the root */208  style?: React.CSSProperties;209}210 211const SECTIONS = [212  {213    eyebrow: "01 / Inertia",214    title: "Scrolling that glides.",215    body: "Every wheel tick moves a target, not the page. Each frame the viewport eases a fraction of the remaining distance toward it, so motion starts crisp and settles softly.",216  },217  {218    eyebrow: "02 / Interpolation",219    title: "One number, endless feel.",220    body: "Lerp controls the personality. Small values float like a magazine on ice; larger values feel tight and precise. Velocity and progress fall out of the same loop for free.",221  },222  {223    eyebrow: "03 / Respect",224    title: "Native where it matters.",225    body: "Touch keeps its own physics, keyboard and scrollbar stay instant, and at the edges the wheel is released so the surrounding page carries on scrolling.",226  },227  {228    eyebrow: "04 / Restraint",229    title: "Idle when settled.",230    body: "The animation loop only runs while there is distance left to cover. Once the viewport arrives, nothing ticks until the next gesture.",231  },232];233 234export function SmoothScroll({235  children,236  height = 380,237  borderRadius = 20,238  lerp = 0.1,239  wheelMultiplier = 1,240  smooth = true,241  showProgress = true,242  progressColor = "#a78bfa",243  showVelocity = true,244  ariaLabel = "Smooth scrolling content",245  onScroll,246  className = "",247  style,248}: SmoothScrollProps) {249  const reduced = usePrefersReducedMotion();250  const rootRef = React.useRef<HTMLDivElement>(null);251  const viewportRef = React.useRef<HTMLDivElement>(null);252  const readoutRef = React.useRef<HTMLSpanElement>(null);253 254  const handleScroll = React.useCallback(255    (state: SmoothScrollState) => {256      const root = rootRef.current;257      if (root) {258        root.style.setProperty("--mf-ss-p", state.progress.toFixed(4));259        root.style.setProperty("--mf-ss-v", Math.min(1, Math.abs(state.velocity) / 40).toFixed(3));260      }261      const readout = readoutRef.current;262      if (readout) {263        const v = Math.abs(state.velocity);264        const arrow = v < 0.05 ? "·" : state.velocity > 0 ? "↓" : "↑";265        readout.textContent = `${arrow} ${v.toFixed(1)} px/f`;266      }267      onScroll?.({ scroll: state.scroll, limit: state.limit, progress: state.progress, velocity: state.velocity });268    },269    [onScroll],270  );271 272  useSmoothScroll(viewportRef, { lerp, wheelMultiplier, smooth: smooth && !reduced, onScroll: handleScroll });273 274  const vars = {275    "--mf-ss-h": `${height}px`,276    "--mf-ss-r": `${borderRadius}px`,277    "--mf-ss-accent": progressColor,278    ...style,279  } as React.CSSProperties;280 281  return (282    <div ref={rootRef} style={vars} className={`${styles.root} ${className}`.trim()}>283      <div ref={viewportRef} className={styles.viewport} tabIndex={0} role="region" aria-label={ariaLabel}>284        <div className={styles.content}>285          {children ?? (286            <>287              <header className={styles.hero}>288                <span className={styles.kicker}>Smooth Scroll</span>289                <h3 className={styles.heroTitle}>Weightless, not floaty.</h3>290                <p className={styles.heroText}>Scroll inside this panel with a wheel or trackpad.</p>291              </header>292              {SECTIONS.map((section, i) => (293                <section key={section.title} className={styles.section}>294                  <div className={styles.sectionText}>295                    <span className={styles.eyebrow}>{section.eyebrow}</span>296                    <h4 className={styles.sectionTitle}>{section.title}</h4>297                    <p className={styles.sectionBody}>{section.body}</p>298                  </div>299                  <div300                    aria-hidden="true"301                    className={styles.art}302                    style={{ "--mf-ss-i": i } as React.CSSProperties}303                  />304                </section>305              ))}306              <footer className={styles.footer}>307                <span className={styles.footerLine} aria-hidden="true" />308                <p>End of the line. Keep scrolling to hand off to the page.</p>309              </footer>310            </>311          )}312        </div>313      </div>314      {showProgress && (315        <div aria-hidden="true" className={styles.progress}>316          <span className={styles.progressFill} />317        </div>318      )}319      {showVelocity && (320        <div aria-hidden="true" className={styles.badge}>321          <span className={styles.badgeLabel}>Velocity</span>322          <span ref={readoutRef} className={styles.badgeValue}>323            {"· 0.0 px/f"}324          </span>325          <span className={styles.meter}>326            <span className={styles.meterFill} />327          </span>328        </div>329      )}330    </div>331  );332}