06Gravity settle — on scroll entry, an element's position value falls under a gravity value and settles after bouncing off a floor value

<main class="stage">
<div class="gsc is-demo" id="gsc">
  <div class="gsc__viewport" id="gscViewport" role="application" aria-label="Drop-and-settle card list" tabindex="0">
    <div class="gsc__pad-top"></div>
    <div class="gsc__card" data-card>Card 1</div>
    <div class="gsc__spacer"></div>
    <div class="gsc__card" data-card>Card 2</div>
    <div class="gsc__spacer"></div>
    <div class="gsc__card" data-card>Card 3</div>
    <div class="gsc__pad-bottom"></div>
  </div>
  <p class="gsc__hint">Scroll until a card comes into view — it drops under a gravity value, bounces off the floor, and settles.</p>
</div>
</main>

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

// 계산 겹(중력 낙하·반동 적분)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-orange);

.stage { position: relative; }

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

.gsc__viewport {
  position: relative;
  width: 220px;
  height: 150px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: $r-card;
  background: rgba($stage-paper, .9);
  border: 1px solid rgba($subject-ink, .12);
  touch-action: pan-y;
  overscroll-behavior: contain;
  scrollbar-width: thin;
}

.gsc__pad-top { height: 24px; }
.gsc__pad-bottom { height: 24px; }
.gsc__spacer { height: 84px; }

.gsc__card {
  margin: 0 $sp-3;
  height: 48px;
  border-radius: $r-control;
  background: $subject-pink;
  color: $subject-ink;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 13px;
  font-weight: 600;
  box-shadow: $shadow-raised;
  opacity: 0;
  transform: translateY(-40px);
  will-change: transform, opacity;
}

.gsc__hint {
  color: rgba($subject-ink, .72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .gsc { gap: $sp-2; }
  .gsc__viewport { width: 176px; height: 90px; }
  .gsc__card { height: 34px; }
  .gsc__spacer { height: 48px; }
  .gsc__hint { font-size: 11px; }
}

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

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.stage {
  position: relative;
}

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

.gsc__viewport {
  position: relative;
  width: 220px;
  height: 150px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.9);
  border: 1px solid rgba(23, 20, 26, 0.12);
  touch-action: pan-y;
  overscroll-behavior: contain;
  scrollbar-width: thin;
}

.gsc__pad-top {
  height: 24px;
}

.gsc__pad-bottom {
  height: 24px;
}

.gsc__spacer {
  height: 84px;
}

.gsc__card {
  margin: 0 12px;
  height: 48px;
  border-radius: 12px;
  background: #ff9ebb;
  color: #17141a;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 13px;
  font-weight: 600;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  opacity: 0;
  transform: translateY(-40px);
  will-change: transform, opacity;
}

.gsc__hint {
  color: rgba(23, 20, 26, 0.72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .gsc {
    gap: 8px;
  }
  .gsc__viewport {
    width: 176px;
    height: 90px;
  }
  .gsc__card {
    height: 34px;
  }
  .gsc__spacer {
    height: 48px;
  }
  .gsc__hint {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .gsc__card {
    transition: none !important;
    animation: none !important;
  }
}

(function () {
  var root = document.getElementById('gsc');
  var viewport = document.getElementById('gscViewport');
  var cards = Array.prototype.slice.call(document.querySelectorAll('[data-card]'));
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 카드가 뷰포트에 들어오는 순간 위치 값(y)이 START_Y 에서 GRAVITY 가속으로 떨어지다
  // FLOOR_Y(0) 에 닿으면 속도 값에 RESTITUTION 을 곱한 만큼만 되튀어(반동), STOP_VEL 밑이면 멈춘다.
  var START_Y = -40;
  var FLOOR_Y = 0;
  var GRAVITY = 0.85;
  var RESTITUTION = 0.42;
  var STOP_VEL = 0.6;

  function dropTick(card, y, vel) {
    vel += GRAVITY;
    y += vel;
    if (y >= FLOOR_Y) {
      y = FLOOR_Y;
      vel = -vel * RESTITUTION;
      if (Math.abs(vel) < STOP_VEL) {
        card.style.transform = 'translateY(0px)';
        card.dataset.done = '1';
        return;
      }
    }
    card.style.transform = 'translateY(' + y.toFixed(1) + 'px)';
    requestAnimationFrame(function () { dropTick(card, y, vel); });
  }
  function settle(card) {
    if (card.dataset.done) { return; }
    card.style.opacity = '1';
    if (reduce) { card.style.transform = 'translateY(0px)'; card.dataset.done = '1'; return; }
    card.dataset.done = '1';
    dropTick(card, START_Y, 0);
  }

  var io = new IntersectionObserver(function (entries) {
    entries.forEach(function (en) { if (en.isIntersecting) { settle(en.target); } });
  }, { root: viewport, threshold: 0.6 });
  cards.forEach(function (c) { io.observe(c); });

  // 프로그램이 scrollTop 을 옮길 때도 네이티브 scroll 이벤트가 뜬다 — 자동 루프 중임을 이 플래그로
  // 표시해 "진짜 사용자 스크롤"과 구분한다(안 하면 자동 루프 첫 걸음에서 곧장 스스로를 멈춘다).
  var autoScrolling = false;

  function wireReal() {
    viewport.addEventListener('scroll', function () {
      if (autoScrolling) { return; }
      root.classList.remove('is-demo'); stopAutoplay();
    }, { passive: true });
    viewport.addEventListener('keydown', function (e) {
      if (e.key !== 'ArrowDown' && e.key !== 'ArrowUp') { return; }
      root.classList.remove('is-demo'); stopAutoplay();
      viewport.scrollTop += (e.key === 'ArrowDown' ? 40 : -40);
    });
  }
  wireReal();

  // 자동 루프: 스크롤 값을 아래로 서서히 옮겨 카드마다 진입 시점의 낙하 정착을 반복 재현한다.
  var autoTimer = null, raf2 = null;
  function autoStep() {
    var max = viewport.scrollHeight - viewport.clientHeight;
    var next = viewport.scrollTop + 1.6;
    autoScrolling = true;
    if (next >= max) {
      viewport.scrollTop = max;
      raf2 = null;
      setTimeout(function () { if (root.classList.contains('is-demo')) { cards.forEach(function (c) { delete c.dataset.done; c.style.transform = 'translateY(' + START_Y + 'px)'; c.style.opacity = '0'; }); autoScrolling = true; viewport.scrollTop = 0; } }, 500);
      return;
    }
    viewport.scrollTop = next;
    raf2 = requestAnimationFrame(autoStep);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } if (raf2) { cancelAnimationFrame(raf2); raf2 = null; } }
  if (!reduce) { autoTimer = setInterval(function () { if (raf2 === null) { raf2 = requestAnimationFrame(autoStep); } }, 1800); autoTimer && requestAnimationFrame(autoStep); }
})();

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

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

export default function ScrollMomentumGravitySettleCard({
  color = "#f9a8d4",
}: ScrollMomentumGravitySettleCardProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const viewport = root.querySelector<HTMLDivElement>(`.${styles.viewport}`);
    const cards = Array.from(root.querySelectorAll<HTMLDivElement>(`.${styles.card}`));
    if (!viewport || cards.length === 0) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    // 계산 겹: 카드가 뷰포트에 들어오는 순간 위치 값(y)이 START_Y 에서 GRAVITY 가속으로 떨어지다
    // FLOOR_Y(0) 에 닿으면 속도 값에 RESTITUTION 을 곱한 만큼만 되튀어(반동), STOP_VEL 밑이면 멈춘다.
    const START_Y = -40;
    const FLOOR_Y = 0;
    const GRAVITY = 0.85;
    const RESTITUTION = 0.42;
    const STOP_VEL = 0.6;

    function dropTick(card: HTMLDivElement, y: number, vel: number) {
      vel += GRAVITY;
      y += vel;
      if (y >= FLOOR_Y) {
        y = FLOOR_Y;
        vel = -vel * RESTITUTION;
        if (Math.abs(vel) < STOP_VEL) {
          card.style.transform = "translateY(0px)";
          card.dataset.done = "1";
          return;
        }
      }
      card.style.transform = "translateY(" + y.toFixed(1) + "px)";
      requestAnimationFrame(() => dropTick(card, y, vel));
    }
    function settle(card: HTMLDivElement) {
      if (card.dataset.done) return;
      card.style.opacity = "1";
      if (reduce) { card.style.transform = "translateY(0px)"; card.dataset.done = "1"; return; }
      card.dataset.done = "1";
      dropTick(card, START_Y, 0);
    }

    const io = new IntersectionObserver((entries) => {
      entries.forEach((en) => { if (en.isIntersecting) settle(en.target as HTMLDivElement); });
    }, { root: viewport, threshold: 0.6 });
    cards.forEach((c) => io.observe(c));

    // 프로그램이 scrollTop 을 옮길 때도 네이티브 scroll 이벤트가 뜬다 — 자동 루프 중임을 이 플래그로
    // 표시해 "진짜 사용자 스크롤"과 구분한다.
    let autoScrolling = false;
    function onScroll() {
      if (autoScrolling) return;
      root!.classList.remove(styles.isDemo);
      stopAutoplay();
    }
    function onKeydown(e: KeyboardEvent) {
      if (e.key !== "ArrowDown" && e.key !== "ArrowUp") return;
      root!.classList.remove(styles.isDemo);
      stopAutoplay();
      viewport!.scrollTop += e.key === "ArrowDown" ? 40 : -40;
    }
    viewport.addEventListener("scroll", onScroll, { passive: true });
    viewport.addEventListener("keydown", onKeydown);

    let autoTimer: number | null = null;
    let raf2: number | null = null;
    function autoStep() {
      const max = viewport!.scrollHeight - viewport!.clientHeight;
      const next = viewport!.scrollTop + 1.6;
      autoScrolling = true;
      if (next >= max) {
        viewport!.scrollTop = max;
        raf2 = null;
        window.setTimeout(() => {
          if (root!.classList.contains(styles.isDemo)) {
            cards.forEach((c) => { delete c.dataset.done; c.style.transform = "translateY(" + START_Y + "px)"; c.style.opacity = "0"; });
            autoScrolling = true;
            viewport!.scrollTop = 0;
          }
        }, 500);
        return;
      }
      viewport!.scrollTop = next;
      raf2 = requestAnimationFrame(autoStep);
    }
    function stopAutoplay() {
      if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; }
      if (raf2 !== null) { cancelAnimationFrame(raf2); raf2 = null; }
    }
    if (!reduce) {
      autoTimer = window.setInterval(() => { if (raf2 === null) raf2 = requestAnimationFrame(autoStep); }, 1800);
      raf2 = requestAnimationFrame(autoStep);
    }

    return () => {
      io.disconnect();
      stopAutoplay();
      viewport.removeEventListener("scroll", onScroll);
      viewport.removeEventListener("keydown", onKeydown);
    };
  }, []);

  return (
    <div ref={rootRef} className={`${styles.stage} ${styles.isDemo}`}>
      <div className={styles.gsc}>
        <div
          className={styles.viewport}
          role="application"
          aria-label="낙하 정착 카드 목록"
          tabIndex={0}
        >
          <div className={styles.padTop} />
          <div className={styles.card}>카드 1</div>
          <div className={styles.spacer} />
          <div className={styles.card} style={{ background: color }}>카드 2</div>
          <div className={styles.spacer} />
          <div className={styles.card}>카드 3</div>
          <div className={styles.padBottom} />
        </div>
        <p className={styles.hint}>스크롤해 카드가 보이면 중력 값으로 떨어지다 바닥에서 튕기며 정착합니다</p>
      </div>
    </div>
  );
}