02Overscroll elasticity — past the scroll boundary, a stretched value springs back

<main class="stage">
<div class="eoh is-demo" id="eoh">
  <div class="eoh__frame" id="eohFrame" role="application" aria-label="Overscroll header test panel">
    <div class="eoh__header" id="eohHeader">Header</div>
    <div class="eoh__body" id="eohBody">
      <div class="eoh__row">Alert 01</div>
      <div class="eoh__row">Alert 02</div>
      <div class="eoh__row">Alert 03</div>
    </div>
  </div>
  <p class="eoh__hint">Pull down from the top and release — the header stretches, then springs back.</p>
</div>
</main>

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

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

.stage { position: relative; }

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

.eoh__frame {
  position: relative;
  width: 210px;
  height: 168px;
  overflow: hidden;
  border-radius: $r-card;
  background: $stage-paper;
  box-shadow: $shadow-raised;
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
}

.eoh__header {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 56px;
  transform-origin: top center;
  display: flex;
  align-items: flex-end;
  justify-content: center;
  padding-bottom: $sp-2;
  background: $subject-lilac;
  color: $subject-ink;
  font-size: 14px;
  font-weight: 700;
  will-change: transform;
}

.eoh__body {
  position: relative;
  margin-top: 56px;
  padding: $sp-2 $sp-3;
  display: flex;
  flex-direction: column;
  gap: $sp-2;
  will-change: transform;
}

.eoh__row {
  height: 26px;
  flex: 0 0 auto;
  display: flex;
  align-items: center;
  padding-left: $sp-2;
  border-radius: $r-sm;
  background: rgba($subject-ink, .06);
  color: $subject-ink;
  font-size: 12px;
}

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

@media (max-width: 340px) {
  .eoh { gap: $sp-2; }
  .eoh__frame { width: 160px; height: 96px; }
  .eoh__header { height: 36px; font-size: 12px; }
  .eoh__body { margin-top: 36px; }
  .eoh__row { height: 18px; font-size: 11px; }
  .eoh__hint { font-size: 11px; }
}

@media (prefers-reduced-motion: reduce) {
  .eoh__header, .eoh__body { 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: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.stage {
  position: relative;
}

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

.eoh__frame {
  position: relative;
  width: 210px;
  height: 168px;
  overflow: hidden;
  border-radius: 16px;
  background: #ffffff;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
}

.eoh__header {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 56px;
  transform-origin: top center;
  display: flex;
  align-items: flex-end;
  justify-content: center;
  padding-bottom: 8px;
  background: #b9a0ff;
  color: #17141a;
  font-size: 14px;
  font-weight: 700;
  will-change: transform;
}

.eoh__body {
  position: relative;
  margin-top: 56px;
  padding: 8px 12px;
  display: flex;
  flex-direction: column;
  gap: 8px;
  will-change: transform;
}

.eoh__row {
  height: 26px;
  flex: 0 0 auto;
  display: flex;
  align-items: center;
  padding-left: 8px;
  border-radius: 8px;
  background: rgba(23, 20, 26, 0.06);
  color: #17141a;
  font-size: 12px;
}

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

@media (max-width: 340px) {
  .eoh {
    gap: 8px;
  }
  .eoh__frame {
    width: 160px;
    height: 96px;
  }
  .eoh__header {
    height: 36px;
    font-size: 12px;
  }
  .eoh__body {
    margin-top: 36px;
  }
  .eoh__row {
    height: 18px;
    font-size: 11px;
  }
  .eoh__hint {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .eoh__header, .eoh__body {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('eoh');
  var frame = document.getElementById('eohFrame');
  var header = document.getElementById('eohHeader');
  var body = document.getElementById('eohBody');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 당긴 거리(dist)는 제곱근으로 저항을 걸어 늘어난 값(stretch)을 만든다(고무줄 저항).
  // 손을 놓으면 stretch 를 목표 1 로 스프링 적분(SPRING_K·SPRING_C)해 되돌린다 — 속성 트윈이 아니라
  // 매 프레임 가속도를 계산해 쌓는 진짜 스프링.
  var PULL_RESIST = 9;
  var MAX_STRETCH = 1.85;
  var SPRING_K = 210;
  var SPRING_C = 15;
  var HEADER_H = 56;

  var stretch = 1, sVel = 0, raf = null;

  function paint() {
    header.style.transform = 'scaleY(' + stretch.toFixed(3) + ')';
    var extra = HEADER_H * (stretch - 1);
    body.style.transform = 'translateY(' + extra.toFixed(1) + 'px)';
  }
  function stopSpring() { if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
  function springTick() {
    var force = -SPRING_K * (stretch - 1) - SPRING_C * sVel;
    sVel += force * (1 / 60) * 0.06;
    stretch += sVel;
    paint();
    if (Math.abs(stretch - 1) < 0.003 && Math.abs(sVel) < 0.003) {
      stretch = 1; sVel = 0; paint(); raf = null; return;
    }
    raf = requestAnimationFrame(springTick);
  }
  function release() {
    if (reduce) { stretch = 1; sVel = 0; paint(); return; }
    stopSpring();
    raf = requestAnimationFrame(springTick);
  }
  function pullTo(dist) {
    var d = Math.max(0, dist);
    stretch = Math.min(MAX_STRETCH, 1 + Math.sqrt(d) / PULL_RESIST);
    paint();
  }

  function wireReal() {
    var dragging = false, startY = 0;
    frame.addEventListener('pointerdown', function (e) {
      root.classList.remove('is-demo'); stopAutoplay(); stopSpring();
      dragging = true; startY = e.clientY; sVel = 0;
      frame.setPointerCapture(e.pointerId);
    });
    frame.addEventListener('pointermove', function (e) {
      if (!dragging) { return; }
      pullTo(e.clientY - startY);
    });
    frame.addEventListener('pointerup', function () {
      if (!dragging) { return; }
      dragging = false;
      release();
    });
    frame.addEventListener('wheel', function (e) {
      if (e.deltaY >= 0) { return; }
      e.preventDefault();
      root.classList.remove('is-demo'); stopAutoplay(); stopSpring();
      pullTo((stretch - 1) * PULL_RESIST * PULL_RESIST + (-e.deltaY) * 1.4);
      release();
    }, { passive: false });
  }
  wireReal();
  paint();

  // 자동 루프: 당김 거리를 늘렸다가 놓기를 반복해 같은 저항→스프링 경로를 재현한다.
  var autoTimer = null, phase = 0;
  function autoTick() {
    phase += 1;
    var d = 58 + Math.sin(phase * 1.1) * 20;
    pullTo(d);
    setTimeout(function () { if (!root.classList.contains('is-demo')) { return; } release(); }, 260);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 900); autoTick(); }
})();

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

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

export default function ScrollMomentumElasticOverscrollHeader({
  color = "#b9a0ff",
}: ScrollMomentumElasticOverscrollHeaderProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

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

    // 계산 겹: 당긴 거리(dist)는 제곱근으로 저항을 걸어 늘어난 값(stretch)을 만든다(고무줄 저항).
    // 손을 놓으면 stretch 를 목표 1 로 스프링 적분(SPRING_K·SPRING_C)해 되돌린다.
    const PULL_RESIST = 9;
    const MAX_STRETCH = 1.85;
    const SPRING_K = 210;
    const SPRING_C = 15;
    const HEADER_H = 56;

    let stretch = 1, sVel = 0, raf: number | null = null;

    function paint() {
      header!.style.transform = "scaleY(" + stretch.toFixed(3) + ")";
      const extra = HEADER_H * (stretch - 1);
      body!.style.transform = "translateY(" + extra.toFixed(1) + "px)";
    }
    function stopSpring() { if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
    function springTick() {
      const force = -SPRING_K * (stretch - 1) - SPRING_C * sVel;
      sVel += force * (1 / 60) * 0.06;
      stretch += sVel;
      paint();
      if (Math.abs(stretch - 1) < 0.003 && Math.abs(sVel) < 0.003) {
        stretch = 1; sVel = 0; paint(); raf = null; return;
      }
      raf = requestAnimationFrame(springTick);
    }
    function release() {
      if (reduce) { stretch = 1; sVel = 0; paint(); return; }
      stopSpring();
      raf = requestAnimationFrame(springTick);
    }
    function pullTo(dist: number) {
      const d = Math.max(0, dist);
      stretch = Math.min(MAX_STRETCH, 1 + Math.sqrt(d) / PULL_RESIST);
      paint();
    }

    let dragging = false, startY = 0;
    function onPointerDown(e: PointerEvent) {
      stopAutoplay(); stopSpring();
      dragging = true; startY = e.clientY; sVel = 0;
      frame!.setPointerCapture(e.pointerId);
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      pullTo(e.clientY - startY);
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false;
      release();
    }
    function onWheel(e: WheelEvent) {
      if (e.deltaY >= 0) return;
      e.preventDefault();
      stopAutoplay(); stopSpring();
      pullTo((stretch - 1) * PULL_RESIST * PULL_RESIST + -e.deltaY * 1.4);
      release();
    }

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

    let autoTimer: number | null = null, phase = 0;
    let releaseTimeout: number | null = null;
    function autoTick() {
      phase += 1;
      const d = 58 + Math.sin(phase * 1.1) * 20;
      pullTo(d);
      releaseTimeout = window.setTimeout(() => { release(); }, 260);
    }
    function stopAutoplay() {
      if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; }
      if (releaseTimeout !== null) { window.clearTimeout(releaseTimeout); releaseTimeout = null; }
    }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 900); autoTick(); }

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

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.eoh}>
        <div className={styles.frame} role="application" aria-label="오버스크롤 헤더 실험판">
          <div className={styles.header} style={{ background: color }}>헤더</div>
          <div className={styles.body}>
            <div className={styles.row}>알림 01</div>
            <div className={styles.row}>알림 02</div>
            <div className={styles.row}>알림 03</div>
          </div>
        </div>
        <p className={styles.hint}>위에서 아래로 당겼다 놓으면 헤더가 늘어났다 되돌아옵니다</p>
      </div>
    </div>
  );
}