09Rubber pull — the moment scroll direction reverses, a value pulls like a rubber band and springs back

<main class="stage">
<div class="rpn is-demo" id="rpn">
  <div class="rpn__frame">
    <div class="rpn__nav" id="rpnNav"><span>Section guide</span></div>
    <div class="rpn__viewport" id="rpnViewport" role="application" aria-label="Direction-reversal rubber nav test panel" tabindex="0">
      <div class="rpn__track" id="rpnTrack">
        <div class="rpn__row">Section 1</div>
        <div class="rpn__row">Section 2</div>
        <div class="rpn__row">Section 3</div>
        <div class="rpn__row">Section 4</div>
        <div class="rpn__row">Section 5</div>
        <div class="rpn__row">Section 6</div>
      </div>
    </div>
  </div>
  <p class="rpn__hint">Every time you reverse scroll direction, the nav bar snaps back like a rubber band.</p>
</div>
</main>

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

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

.stage { position: relative; }

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

.rpn__frame {
  width: 220px;
  border-radius: $r-card;
  overflow: hidden;
  background: rgba($subject-ink, .06);
  border: 1px solid rgba($subject-ink, .14);
}

.rpn__nav {
  height: 34px;
  background: $subject-blue-deep;
  color: $stage-paper;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 12px;
  font-weight: 700;
  transform-origin: center top;
  will-change: transform;
}

.rpn__viewport {
  position: relative;
  width: 100%;
  height: 150px;
  overflow: hidden;
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
  box-sizing: border-box;
}

.rpn__track {
  position: absolute;
  inset: 0;
  will-change: transform;
}

.rpn__row {
  height: 40px;
  margin: 0 $sp-3;
  border-radius: $r-control;
  background: rgba($subject-ink, .07);
  color: $subject-ink;
  display: flex;
  align-items: center;
  padding-left: $sp-2;
  font-size: 12px;
  box-sizing: border-box;
}

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

@media (max-width: 340px) {
  .rpn { gap: $sp-2; }
  .rpn__frame { width: 176px; }
  .rpn__nav { height: 26px; font-size: 11px; }
  .rpn__viewport { height: 80px; }
  .rpn__row { height: 30px; }
  .rpn__hint { font-size: 11px; }
}

@media (prefers-reduced-motion: reduce) {
  .rpn__nav { 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;
}

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

.rpn__frame {
  width: 220px;
  border-radius: 16px;
  overflow: hidden;
  background: rgba(23, 20, 26, 0.06);
  border: 1px solid rgba(23, 20, 26, 0.14);
}

.rpn__nav {
  height: 34px;
  background: #0a4fe8;
  color: #ffffff;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 12px;
  font-weight: 700;
  transform-origin: center top;
  will-change: transform;
}

.rpn__viewport {
  position: relative;
  width: 100%;
  height: 150px;
  overflow: hidden;
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
  box-sizing: border-box;
}

.rpn__track {
  position: absolute;
  inset: 0;
  will-change: transform;
}

.rpn__row {
  height: 40px;
  margin: 0 12px;
  border-radius: 12px;
  background: rgba(23, 20, 26, 0.07);
  color: #17141a;
  display: flex;
  align-items: center;
  padding-left: 8px;
  font-size: 12px;
  box-sizing: border-box;
}

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

@media (max-width: 340px) {
  .rpn {
    gap: 8px;
  }
  .rpn__frame {
    width: 176px;
  }
  .rpn__nav {
    height: 26px;
    font-size: 11px;
  }
  .rpn__viewport {
    height: 80px;
  }
  .rpn__row {
    height: 30px;
  }
  .rpn__hint {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .rpn__nav {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('rpn');
  var viewport = document.getElementById('rpnViewport');
  var track = document.getElementById('rpnTrack');
  var nav = document.getElementById('rpnNav');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹 ①: 목록은 입력에 1:1 로 즉시 따라간다(scrollPos, 경계 clamp).
  var rowH = 40, rowCount = 6;
  var maxScroll = Math.max(0, rowH * rowCount - viewport.clientHeight);
  var scrollPos = 0;
  var lastVel = 0;

  // 계산 겹 ②: 스크롤 속도(vel)의 부호가 바뀌는 순간(방향 전환)에만 네비 바에 충격을 주고,
  // 그 뒤엔 스프링 적분(SPRING_K·SPRING_C)이 scale 을 1로 되돌리며 튕긴다 — 경계에 닿아서가 아니다.
  var MIN_VEL = 1.2;
  var KICK = 0.4;
  var SPRING_K = 260;
  var SPRING_C = 14;
  var navScale = 1, navVel = 0;
  var navRaf = null;

  function paintTrack() { track.style.transform = 'translateY(-' + scrollPos.toFixed(1) + 'px)'; }
  function paintNav() { nav.style.transform = 'scaleY(' + navScale.toFixed(3) + ')'; }
  function navTick() {
    var force = -SPRING_K * (navScale - 1) - SPRING_C * navVel;
    navVel += force * 0.016;
    navScale += navVel * 0.016;
    if (reduce) { navScale = 1; navVel = 0; paintNav(); navRaf = null; return; }
    if (Math.abs(navScale - 1) < 0.003 && Math.abs(navVel) < 0.02) {
      navScale = 1; navVel = 0; paintNav(); navRaf = null; return;
    }
    paintNav();
    navRaf = requestAnimationFrame(navTick);
  }
  function ensureNavLoop() { if (navRaf === null) { navRaf = requestAnimationFrame(navTick); } }
  function kickNav(sign) {
    if (reduce) { return; }
    navVel += sign * KICK;
    ensureNavLoop();
  }
  function applyVel(vel) {
    if (lastVel !== 0 && vel !== 0 && (lastVel > 0) !== (vel > 0) && Math.abs(lastVel) > MIN_VEL && Math.abs(vel) > MIN_VEL) {
      kickNav(vel > 0 ? 1 : -1);
    }
    if (Math.abs(vel) > 0.1) { lastVel = vel; }
  }
  function setScroll(v, vel) {
    var next = Math.max(0, Math.min(maxScroll, v));
    applyVel(vel);
    scrollPos = next;
    paintTrack();
  }
  paintTrack(); paintNav();

  function wireReal() {
    viewport.addEventListener('wheel', function (e) {
      e.preventDefault();
      root.classList.remove('is-demo'); stopAutoplay();
      setScroll(scrollPos + e.deltaY, e.deltaY);
    }, { passive: false });

    var dragging = false, lastY = 0;
    viewport.addEventListener('pointerdown', function (e) {
      root.classList.remove('is-demo'); stopAutoplay();
      dragging = true; lastY = e.clientY;
      viewport.setPointerCapture(e.pointerId);
    });
    viewport.addEventListener('pointermove', function (e) {
      if (!dragging) { return; }
      var dy = -(e.clientY - lastY);
      setScroll(scrollPos + dy, dy);
      lastY = e.clientY;
    });
    viewport.addEventListener('pointerup', function () { dragging = false; });
    viewport.addEventListener('keydown', function (e) {
      if (e.key !== 'ArrowDown' && e.key !== 'ArrowUp') { return; }
      root.classList.remove('is-demo'); stopAutoplay();
      var dy = (e.key === 'ArrowDown' ? 40 : -40);
      setScroll(scrollPos + dy, dy);
    });
  }
  wireReal();

  // 자동 루프: 아래로 갔다가 위로 되짚어 방향 전환을 주기적으로 만든다.
  var autoTimer = null, phase = 0;
  function autoTick() {
    phase += 1;
    var dir = (phase % 2 === 0) ? -1 : 1;
    setScroll(scrollPos + dir * 60, dir * 60);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 900); autoTick(); }
})();

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

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

export default function ScrollMomentumRubberPullNav({
  color = "#2f6df6",
}: ScrollMomentumRubberPullNavProps) {
  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}`);
    const nav = root.querySelector<HTMLDivElement>(`.${styles.nav}`);
    if (!viewport || !track || !nav) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    // 계산 겹 ①: 목록은 입력에 1:1 로 즉시 따라간다(scrollPos, 경계 clamp).
    const rowH = 40, rowCount = 6;
    const maxScroll = Math.max(0, rowH * rowCount - viewport.clientHeight);
    let scrollPos = 0;
    let lastVel = 0;

    // 계산 겹 ②: 스크롤 속도(vel)의 부호가 바뀌는 순간(방향 전환)에만 네비 바에 충격을 주고,
    // 그 뒤엔 스프링 적분(SPRING_K·SPRING_C)이 scale 을 1로 되돌리며 튕긴다 — 경계에 닿아서가 아니다.
    const MIN_VEL = 1.2;
    const KICK = 0.4;
    const SPRING_K = 260;
    const SPRING_C = 14;
    let navScale = 1, navVel = 0;
    let navRaf: number | null = null;

    function paintTrack() { track!.style.transform = "translateY(-" + scrollPos.toFixed(1) + "px)"; }
    function paintNav() { nav!.style.transform = "scaleY(" + navScale.toFixed(3) + ")"; }
    function navTick() {
      const force = -SPRING_K * (navScale - 1) - SPRING_C * navVel;
      navVel += force * 0.016;
      navScale += navVel * 0.016;
      if (reduce) { navScale = 1; navVel = 0; paintNav(); navRaf = null; return; }
      if (Math.abs(navScale - 1) < 0.003 && Math.abs(navVel) < 0.02) {
        navScale = 1; navVel = 0; paintNav(); navRaf = null; return;
      }
      paintNav();
      navRaf = requestAnimationFrame(navTick);
    }
    function ensureNavLoop() { if (navRaf === null) navRaf = requestAnimationFrame(navTick); }
    function kickNav(sign: number) {
      if (reduce) return;
      navVel += sign * KICK;
      ensureNavLoop();
    }
    function applyVel(vel: number) {
      if (lastVel !== 0 && vel !== 0 && (lastVel > 0) !== (vel > 0) && Math.abs(lastVel) > MIN_VEL && Math.abs(vel) > MIN_VEL) {
        kickNav(vel > 0 ? 1 : -1);
      }
      if (Math.abs(vel) > 0.1) lastVel = vel;
    }
    function setScroll(v: number, vel: number) {
      const next = Math.max(0, Math.min(maxScroll, v));
      applyVel(vel);
      scrollPos = next;
      paintTrack();
    }
    paintTrack(); paintNav();

    function onWheel(e: WheelEvent) {
      e.preventDefault();
      stopAutoplay();
      setScroll(scrollPos + e.deltaY, e.deltaY);
    }
    let dragging = false, lastY = 0;
    function onPointerDown(e: PointerEvent) {
      stopAutoplay();
      dragging = true; lastY = e.clientY;
      viewport!.setPointerCapture(e.pointerId);
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const dy = -(e.clientY - lastY);
      setScroll(scrollPos + dy, dy);
      lastY = e.clientY;
    }
    function onPointerUp() { dragging = false; }
    function onKeydown(e: KeyboardEvent) {
      if (e.key !== "ArrowDown" && e.key !== "ArrowUp") return;
      stopAutoplay();
      const dy = e.key === "ArrowDown" ? 40 : -40;
      setScroll(scrollPos + dy, dy);
    }
    viewport.addEventListener("wheel", onWheel, { passive: false });
    viewport.addEventListener("pointerdown", onPointerDown);
    viewport.addEventListener("pointermove", onPointerMove);
    viewport.addEventListener("pointerup", onPointerUp);
    viewport.addEventListener("keydown", onKeydown);

    let autoTimer: number | null = null, phase = 0;
    function autoTick() {
      phase += 1;
      const dir = phase % 2 === 0 ? -1 : 1;
      setScroll(scrollPos + dir * 60, dir * 60);
    }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 900); autoTick(); }

    return () => {
      stopAutoplay();
      if (navRaf !== null) cancelAnimationFrame(navRaf);
      viewport.removeEventListener("wheel", onWheel);
      viewport.removeEventListener("pointerdown", onPointerDown);
      viewport.removeEventListener("pointermove", onPointerMove);
      viewport.removeEventListener("pointerup", onPointerUp);
      viewport.removeEventListener("keydown", onKeydown);
    };
  }, []);

  const rows = [1, 2, 3, 4, 5, 6];

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.rpn}>
        <div className={styles.frame}>
          <div className={styles.nav} style={{ background: color }}><span>구간 안내</span></div>
          <div className={styles.viewport} role="application" aria-label="방향 전환 러버 네비 실험판" tabIndex={0}>
            <div className={styles.track}>
              {rows.map((n) => (
                <div key={n} className={styles.row}>섹션 {n}</div>
              ))}
            </div>
          </div>
        </div>
        <p className={styles.hint}>스크롤 방향을 바꾸는 순간마다 위 네비 바가 고무줄처럼 튕깁니다</p>
      </div>
    </div>
  );
}