04Section snap spring — the released scroll position value settles toward the nearest section value through spring damping

<main class="stage">
<div class="sss is-demo" id="sss">
  <div class="sss__viewport" id="sssViewport" role="application" aria-label="Section-snap scroll panel">
    <div class="sss__track" id="sssTrack">
      <section class="sss__panel sss__panel--1">Section 1</section>
      <section class="sss__panel sss__panel--2">Section 2</section>
      <section class="sss__panel sss__panel--3">Section 3</section>
    </div>
  </div>
  <p class="sss__hint">Drag to scroll and release — it springs to settle at the nearest section.</p>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;

// 계산 겹(스프링 적분)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-paper);

.stage { position: relative; }

.sss {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
}

.sss__viewport {
  position: relative;
  width: 200px;
  height: 150px;
  overflow: hidden;
  border-radius: $r-card;
  box-shadow: $shadow-raised;
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
}

.sss__track {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  will-change: transform;
}

.sss__panel {
  width: 100%;
  height: 150px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 16px;
  font-weight: 700;
  color: $subject-ink;
  box-sizing: border-box;

  &--1 { background: $subject-mint; }
  &--2 { background: $subject-sky; }
  &--3 { background: $subject-lilac; }
}

.sss__hint {
  color: $subject-ink;
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .sss { gap: $sp-2; }
  .sss__viewport { width: 152px; height: 86px; }
  .sss__panel { height: 86px; font-size: 13px; }
  .sss__hint { font-size: 11px; }
}

@media (prefers-reduced-motion: reduce) {
  .sss__track { transition: none !important; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.stage {
  position: relative;
}

.sss {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

.sss__viewport {
  position: relative;
  width: 200px;
  height: 150px;
  overflow: hidden;
  border-radius: 16px;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
}

.sss__track {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  will-change: transform;
}

.sss__panel {
  width: 100%;
  height: 150px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 16px;
  font-weight: 700;
  color: #17141a;
  box-sizing: border-box;
}
.sss__panel--1 {
  background: #4cd4a6;
}
.sss__panel--2 {
  background: #6ec8ff;
}
.sss__panel--3 {
  background: #b9a0ff;
}

.sss__hint {
  color: #17141a;
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .sss {
    gap: 8px;
  }
  .sss__viewport {
    width: 152px;
    height: 86px;
  }
  .sss__panel {
    height: 86px;
    font-size: 13px;
  }
  .sss__hint {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sss__track {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('sss');
  var viewport = document.getElementById('sssViewport');
  var track = document.getElementById('sssTrack');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 놓인 위치 값(pos)이 가장 가까운 구간 값(target = round(pos/SECTION_H)*SECTION_H)을
  // 향해 스프링 방정식(SPRING_K·SPRING_C)으로 매 프레임 가속도를 쌓아 다가간다 — 목표를 살짝
  // 지나쳤다 되돌아오길 반복하며 잦아든다(과소감쇠 스프링).
  var SECTION_H = 150;
  var SECTION_COUNT = 3;
  var SPRING_K = 150;
  var SPRING_C = 7.5;

  var pos = 0, vel = 0, raf = null, maxScroll = SECTION_H * (SECTION_COUNT - 1);

  function clampFree(v) { return Math.max(-maxScroll, Math.min(0, v)); }
  function paint() { track.style.transform = 'translateY(' + pos.toFixed(1) + 'px)'; }
  function stopSpring() { if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
  function nearestTarget() { return clampFree(Math.round(pos / SECTION_H) * SECTION_H); }

  function springTick(target) {
    var force = -SPRING_K * (pos - target) - SPRING_C * vel;
    vel += force * (1 / 60) * 0.06;
    pos += vel;
    paint();
    if (Math.abs(pos - target) < 0.4 && Math.abs(vel) < 0.4) {
      pos = target; vel = 0; paint(); raf = null; return;
    }
    raf = requestAnimationFrame(function () { springTick(target); });
  }
  function snap() {
    var target = nearestTarget();
    if (reduce) { pos = target; vel = 0; paint(); return; }
    stopSpring();
    raf = requestAnimationFrame(function () { springTick(target); });
  }

  var wheelTimer = null;
  function wireReal() {
    var dragging = false, startY = 0, startPos = 0;
    viewport.addEventListener('pointerdown', function (e) {
      root.classList.remove('is-demo'); stopAutoplay(); stopSpring();
      dragging = true; startY = e.clientY; startPos = pos; vel = 0;
      viewport.setPointerCapture(e.pointerId);
    });
    viewport.addEventListener('pointermove', function (e) {
      if (!dragging) { return; }
      pos = clampFree(startPos + (e.clientY - startY));
      paint();
    });
    viewport.addEventListener('pointerup', function () {
      if (!dragging) { return; }
      dragging = false;
      snap();
    });
    viewport.addEventListener('wheel', function (e) {
      e.preventDefault();
      root.classList.remove('is-demo'); stopAutoplay(); stopSpring();
      pos = clampFree(pos - e.deltaY * 0.5);
      paint();
      if (wheelTimer) { clearTimeout(wheelTimer); }
      wheelTimer = setTimeout(snap, 140);
    }, { passive: false });
  }
  wireReal();
  paint();

  // 자동 루프: 다음 구간으로 자유 이동 값을 옮긴 뒤 스프링 정착을 반복 재현한다.
  var autoTimer = null, section = 0;
  function autoTick() {
    section = (section + 1) % SECTION_COUNT;
    pos = clampFree(-section * SECTION_H + 26);
    paint();
    snap();
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } if (wheelTimer) { clearTimeout(wheelTimer); wheelTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 1300); autoTick(); }
})();

import { useEffect, useRef } from "react";
import styles from "./ScrollMomentumSpringSectionSnap.module.scss";

export type ScrollMomentumSpringSectionSnapProps = {
  color?: string;
};

export default function ScrollMomentumSpringSectionSnap({
  color = "#4cd4a6",
}: ScrollMomentumSpringSectionSnapProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const viewport = root.querySelector<HTMLDivElement>(`.${styles.viewport}`);
    const track = root.querySelector<HTMLDivElement>(`.${styles.track}`);
    if (!viewport || !track) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    // 계산 겹: 놓인 위치 값(pos)이 가장 가까운 구간 값(target = round(pos/SECTION_H)*SECTION_H)을
    // 향해 스프링 방정식(SPRING_K·SPRING_C)으로 매 프레임 가속도를 쌓아 다가간다 — 목표를 살짝
    // 지나쳤다 되돌아오길 반복하며 잦아든다(과소감쇠 스프링).
    const SECTION_H = 150;
    const SECTION_COUNT = 3;
    const SPRING_K = 150;
    const SPRING_C = 7.5;

    let pos = 0, vel = 0, raf: number | null = null;
    const maxScroll = SECTION_H * (SECTION_COUNT - 1);

    function clampFree(v: number) { return Math.max(-maxScroll, Math.min(0, v)); }
    function paint() { track!.style.transform = "translateY(" + pos.toFixed(1) + "px)"; }
    function stopSpring() { if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
    function nearestTarget() { return clampFree(Math.round(pos / SECTION_H) * SECTION_H); }

    function springTick(target: number) {
      const force = -SPRING_K * (pos - target) - SPRING_C * vel;
      vel += force * (1 / 60) * 0.06;
      pos += vel;
      paint();
      if (Math.abs(pos - target) < 0.4 && Math.abs(vel) < 0.4) {
        pos = target; vel = 0; paint(); raf = null; return;
      }
      raf = requestAnimationFrame(() => springTick(target));
    }
    function snap() {
      const target = nearestTarget();
      if (reduce) { pos = target; vel = 0; paint(); return; }
      stopSpring();
      raf = requestAnimationFrame(() => springTick(target));
    }

    let wheelTimer: number | null = null;
    let dragging = false, startY = 0, startPos = 0;
    function onPointerDown(e: PointerEvent) {
      stopAutoplay(); stopSpring();
      dragging = true; startY = e.clientY; startPos = pos; vel = 0;
      viewport!.setPointerCapture(e.pointerId);
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      pos = clampFree(startPos + (e.clientY - startY));
      paint();
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false;
      snap();
    }
    function onWheel(e: WheelEvent) {
      e.preventDefault();
      stopAutoplay(); stopSpring();
      pos = clampFree(pos - e.deltaY * 0.5);
      paint();
      if (wheelTimer !== null) window.clearTimeout(wheelTimer);
      wheelTimer = window.setTimeout(snap, 140);
    }

    viewport.addEventListener("pointerdown", onPointerDown);
    viewport.addEventListener("pointermove", onPointerMove);
    viewport.addEventListener("pointerup", onPointerUp);
    viewport.addEventListener("wheel", onWheel, { passive: false });
    paint();

    let autoTimer: number | null = null, section = 0;
    function autoTick() {
      section = (section + 1) % SECTION_COUNT;
      pos = clampFree(-section * SECTION_H + 26);
      paint();
      snap();
    }
    function stopAutoplay() {
      if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; }
      if (wheelTimer !== null) { window.clearTimeout(wheelTimer); wheelTimer = null; }
    }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 1300); autoTick(); }

    return () => {
      stopAutoplay();
      stopSpring();
      viewport.removeEventListener("pointerdown", onPointerDown);
      viewport.removeEventListener("pointermove", onPointerMove);
      viewport.removeEventListener("pointerup", onPointerUp);
      viewport.removeEventListener("wheel", onWheel);
    };
  }, []);

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.sss}>
        <div className={styles.viewport} role="application" aria-label="구간 스냅 스크롤판">
          <div className={styles.track}>
            <section className={`${styles.panel} ${styles.panel1}`} style={{ background: color }}>구간 1</section>
            <section className={`${styles.panel} ${styles.panel2}`}>구간 2</section>
            <section className={`${styles.panel} ${styles.panel3}`}>구간 3</section>
          </div>
        </div>
        <p className={styles.hint}>드래그로 스크롤한 뒤 놓으면 가까운 구간 값으로 스프링 정착합니다</p>
      </div>
    </div>
  );
}