07Swipe-follow toggle — while a horizontal drag-distance value grows, the highlight's position value follows toward the side tab by that same ratio

<main class="stage">
<div class="sf is-demo" id="sf">
  <div class="sf__phone">
    <div class="sf__track" id="sfTrack">
      <div class="sf__indicator" id="sfIndicator"></div>
      <button type="button" class="sf__tab is-active" data-idx="0">
        <svg class="sf__icon" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M216 120v96h-64v-64h-48v64H40v-96a8 8 0 0 1 2.34-5.66l80-80a8 8 0 0 1 11.32 0l80 80A8 8 0 0 1 216 120" opacity=".2"/><path d="m219.31 108.68l-80-80a16 16 0 0 0-22.62 0l-80 80A15.87 15.87 0 0 0 32 120v96a8 8 0 0 0 8 8h64a8 8 0 0 0 8-8v-56h32v56a8 8 0 0 0 8 8h64a8 8 0 0 0 8-8v-96a15.87 15.87 0 0 0-4.69-11.32M208 208h-48v-56a8 8 0 0 0-8-8h-48a8 8 0 0 0-8 8v56H48v-88l80-80l80 80Z"/></g></svg>
        <span class="sf__label">Home</span>
      </button>
      <button type="button" class="sf__tab" data-idx="1">
        <svg class="sf__icon" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M224 80v120a8 8 0 0 1-8 8H40a8 8 0 0 1-8-8V80a8 8 0 0 1 8-8h176a8 8 0 0 1 8 8" opacity=".2"/><path d="M216 64h-40a48 48 0 0 0-96 0H40a16 16 0 0 0-16 16v120a16 16 0 0 0 16 16h176a16 16 0 0 0 16-16V80a16 16 0 0 0-16-16m-88-32a32 32 0 0 1 32 32H96a32 32 0 0 1 32-32m88 168H40V80h176Z"/></g></svg>
        <span class="sf__label">Shopping</span>
      </button>
      <button type="button" class="sf__tab" data-idx="2">
        <svg class="sf__icon" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M208 192H48a8 8 0 0 1-6.88-12C47.71 168.6 56 139.81 56 104a72 72 0 0 1 144 0c0 35.82 8.3 64.6 14.9 76a8 8 0 0 1-6.9 12" opacity=".2"/><path d="M221.8 175.94c-5.55-9.56-13.8-36.61-13.8-71.94a80 80 0 1 0-160 0c0 35.34-8.26 62.38-13.81 71.94A16 16 0 0 0 48 200h40.81a40 40 0 0 0 78.38 0H208a16 16 0 0 0 13.8-24.06M128 216a24 24 0 0 1-22.62-16h45.24A24 24 0 0 1 128 216m-80-32c7.7-13.24 16-43.92 16-80a64 64 0 1 1 128 0c0 36.05 8.28 66.73 16 80Z"/></g></svg>
        <span class="sf__label">Notifications</span>
      </button>
      <button type="button" class="sf__tab" data-idx="3">
        <svg class="sf__icon" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M192 96a64 64 0 1 1-64-64a64 64 0 0 1 64 64" opacity=".2"/><path d="M230.92 212c-15.23-26.33-38.7-45.21-66.09-54.16a72 72 0 1 0-73.66 0c-27.39 8.94-50.86 27.82-66.09 54.16a8 8 0 1 0 13.85 8c18.84-32.56 52.14-52 89.07-52s70.23 19.44 89.07 52a8 8 0 1 0 13.85-8M72 96a56 56 0 1 1 56 56a56.06 56.06 0 0 1-56-56"/></g></svg>
        <span class="sf__label">Profile</span>
      </button>
    </div>
  </div>
</div>
</main>

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

// 계산 겹: 드래그 중엔 포인터 x 를 그대로 clamp 해 인디케이터가 1:1 로 따라오고,
// 놓으면 가장 가까운 탭의 실측 rect(getBoundingClientRect) 로 스프링(SPRING_K·C) 정착한다.
@include stage($stage-orange);

// .sf 는 is-demo 클래스만 들고 있는 래퍼 — display:contents 로 트리에서 지워
// .sf__phone 이 .stage 그리드의 직계 아이템이 되게 한다(367-05 실측 버그 회피).
.sf { display: contents; }

.sf__phone {
  width: 100%; max-width: 300px; position: relative; height: 96px;
  display: flex; align-items: center;
}
.sf__track {
  position: relative; width: 100%; display: flex; justify-content: space-around;
  background: $subject-cream; border-radius: $r-card; padding: $sp-2 0;
  box-shadow: $shadow-raised; touch-action: none; cursor: grab;
}
.sf__indicator {
  position: absolute; left: 0; top: 8px;
  border-radius: $r-pill; background: $subject-blue-light; will-change: transform;
}
.sf__tab {
  position: relative; z-index: 1; flex: 0 0 auto; display: flex; flex-direction: column;
  align-items: center; gap: $sp-1; border: none; background: transparent; cursor: pointer;
  padding: $sp-1 $sp-3; color: rgba($subject-ink, .72);
}
.sf__tab.is-active { color: $subject-blue-deep; }
.sf__icon { width: 18px; height: 18px; }
.sf__label { font-size: 10px; font-weight: 700; line-height: 1; }

@media (max-width: 340px) {
  .sf__phone { max-width: 230px; }
  .sf__tab { padding: $sp-1 $sp-2; }
}

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

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);
}

.sf {
  display: contents;
}

.sf__phone {
  width: 100%;
  max-width: 300px;
  position: relative;
  height: 96px;
  display: flex;
  align-items: center;
}

.sf__track {
  position: relative;
  width: 100%;
  display: flex;
  justify-content: space-around;
  background: #fff7e6;
  border-radius: 16px;
  padding: 8px 0;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  touch-action: none;
  cursor: grab;
}

.sf__indicator {
  position: absolute;
  left: 0;
  top: 8px;
  border-radius: 999px;
  background: #5b8cf8;
  will-change: transform;
}

.sf__tab {
  position: relative;
  z-index: 1;
  flex: 0 0 auto;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 4px;
  border: none;
  background: transparent;
  cursor: pointer;
  padding: 4px 12px;
  color: rgba(23, 20, 26, 0.72);
}

.sf__tab.is-active {
  color: #0a4fe8;
}

.sf__icon {
  width: 18px;
  height: 18px;
}

.sf__label {
  font-size: 10px;
  font-weight: 700;
  line-height: 1;
}

@media (max-width: 340px) {
  .sf__phone {
    max-width: 230px;
  }
  .sf__tab {
    padding: 4px 8px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sf__indicator {
    transition: none;
  }
}

(function () {
  var SPRING_K = 240; // 계산 겹: 정착(settle) 스프링 상수 — 드래그를 놓은 뒤 가장 가까운 탭으로 당긴다
  var SPRING_C = 20; // 감쇠 상수
  var DRAG_MS = 520; // 자동 루프에서 가상 손가락이 다음 탭까지 이동하는 시간
  var WAIT_MS = 700; // 정착 뒤 머무는 시간
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var sf = document.getElementById('sf');
  var track = document.getElementById('sfTrack');
  var indicator = document.getElementById('sfIndicator');
  var tabs = Array.prototype.slice.call(document.querySelectorAll('.sf__tab'));

  var isDemo = true;
  var geo = []; // 계산 겹: 탭마다 실제 렌더 폭·left 오프셋(getBoundingClientRect 실측)
  var maxX = 0;
  var indW = 0, indH = 0;
  var activeIdx = 0;
  var posX = 0; // 인디케이터 현재 x(px)
  var vel = 0; // 정착 스프링 속도
  var settling = false;
  var settleTarget = 0;
  var dragging = false;
  var dragStartClientX = 0;
  var dragStartX = 0;
  var lastTs = 0;

  function measure() {
    var trackRect = track.getBoundingClientRect();
    geo = tabs.map(function (t) {
      var r = t.getBoundingClientRect();
      return { left: r.left - trackRect.left, width: r.width, height: r.height };
    });
    var first = geo[0];
    indW = first.width; indH = first.height;
    indicator.style.width = indW + 'px';
    indicator.style.height = indH + 'px';
    maxX = geo[geo.length - 1].left;
    posX = geo[activeIdx].left;
    indicator.style.transform = 'translateX(' + posX.toFixed(1) + 'px)';
  }

  function human() {
    if (isDemo) { sf.classList.remove('is-demo'); isDemo = false; }
  }

  function nearestIndex(centerX) {
    var best = 0, bestDist = Infinity;
    geo.forEach(function (g, i) {
      var c = g.left + g.width / 2;
      var d = Math.abs(c - centerX);
      if (d < bestDist) { bestDist = d; best = i; }
    });
    return best;
  }

  function setActive(idx) {
    activeIdx = idx;
    tabs.forEach(function (t, i) { t.classList.toggle('is-active', i === idx); });
  }

  function onPointerDown(e) {
    human();
    dragging = true;
    settling = false;
    dragStartClientX = e.clientX;
    dragStartX = posX;
    track.setPointerCapture && e.pointerId != null && track.setPointerCapture(e.pointerId);
  }
  function onPointerMove(e) {
    if (!dragging) return;
    var dx = e.clientX - dragStartClientX;
    posX = Math.max(0, Math.min(maxX, dragStartX + dx));
    indicator.style.transform = 'translateX(' + posX.toFixed(1) + 'px)';
  }
  function onPointerUp() {
    if (!dragging) return;
    dragging = false;
    var idx = nearestIndex(posX + indW / 2);
    setActive(idx);
    settling = true;
    settleTarget = geo[idx].left;
    vel = 0;
  }

  track.addEventListener('pointerdown', onPointerDown);
  window.addEventListener('pointermove', onPointerMove);
  window.addEventListener('pointerup', onPointerUp);
  window.addEventListener('pointercancel', onPointerUp);

  tabs.forEach(function (t, i) {
    t.addEventListener('click', function () {
      human();
      if (dragging) return;
      setActive(i);
      settling = true;
      settleTarget = geo[i].left;
      vel = 0;
    });
  });

  var demoOrder = [2, 1, 3, 0]; // 자동 루프가 방문할 탭 순서
  var demoStep = 0;
  var demoPhase = 'wait'; // 'drag' | 'settle' | 'wait'
  var demoElapsed = 0;
  var demoFromIdx = 0;

  function tick(ts) {
    if (!lastTs) { lastTs = ts; }
    var dt = Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;

    if (isDemo && !reduce && !dragging) {
      demoElapsed += dt * 1000;
      if (demoPhase === 'drag') {
        var t = Math.min(1, demoElapsed / DRAG_MS);
        var toIdx = demoOrder[demoStep];
        var fromCenter = geo[demoFromIdx].left + indW / 2;
        var toCenter = geo[toIdx].left + indW / 2;
        var virtualCenter = fromCenter + (toCenter - fromCenter) * t;
        posX = Math.max(0, Math.min(maxX, virtualCenter - indW / 2));
        indicator.style.transform = 'translateX(' + posX.toFixed(1) + 'px)';
        if (t >= 1) {
          setActive(toIdx);
          settling = true; settleTarget = geo[toIdx].left; vel = 0;
          demoPhase = 'settle'; demoElapsed = 0;
        }
      } else if (demoPhase === 'settle') {
        if (!settling) {
          demoPhase = 'wait'; demoElapsed = 0;
        }
      } else if (demoPhase === 'wait') {
        if (demoElapsed >= WAIT_MS) {
          demoFromIdx = demoOrder[demoStep];
          demoStep = (demoStep + 1) % demoOrder.length;
          demoPhase = 'drag'; demoElapsed = 0;
        }
      }
    }

    if (settling) {
      var acc = (settleTarget - posX) * SPRING_K - vel * SPRING_C;
      vel += acc * dt;
      posX += vel * dt;
      if (Math.abs(settleTarget - posX) < 0.4 && Math.abs(vel) < 4) {
        posX = settleTarget; vel = 0; settling = false;
      }
      indicator.style.transform = 'translateX(' + posX.toFixed(1) + 'px)';
    }

    requestAnimationFrame(tick);
  }

  measure();
  window.addEventListener('resize', measure);
  requestAnimationFrame(tick);
})();

// SwipeFollow — vanilla 판(run/367/demo/07-bottom-nav-bar-swipe-follow/index.html) 의 React 포트.
// 드래그 중엔 포인터 x 를 그대로 clamp 해 인디케이터가 1:1 로 따라오고, 놓으면 가장 가까운
// 탭의 실측 rect(getBoundingClientRect) 로 스프링(SPRING_K·C) 정착한다.
import { useEffect, useRef } from "react";
import styles from "./SwipeFollow.module.scss";

type Box<T> = { current: T };
type Geo = { left: number; width: number; height: number };

const SPRING_K = 240; // 계산 겹: 정착(settle) 스프링 상수 — 드래그를 놓은 뒤 가장 가까운 탭으로 당긴다
const SPRING_C = 20; // 감쇠 상수
const DRAG_MS = 520; // 자동 루프에서 가상 손가락이 다음 탭까지 이동하는 시간
const WAIT_MS = 700; // 정착 뒤 머무는 시간

const TABS = [
  { label: "홈", d: "M216 120v96h-64v-64h-48v64H40v-96a8 8 0 0 1 2.34-5.66l80-80a8 8 0 0 1 11.32 0l80 80A8 8 0 0 1 216 120|.2#m219.31 108.68l-80-80a16 16 0 0 0-22.62 0l-80 80A15.87 15.87 0 0 0 32 120v96a8 8 0 0 0 8 8h64a8 8 0 0 0 8-8v-56h32v56a8 8 0 0 0 8 8h64a8 8 0 0 0 8-8v-96a15.87 15.87 0 0 0-4.69-11.32M208 208h-48v-56a8 8 0 0 0-8-8h-48a8 8 0 0 0-8 8v56H48v-88l80-80l80 80Z" },
  { label: "쇼핑", d: "M224 80v120a8 8 0 0 1-8 8H40a8 8 0 0 1-8-8V80a8 8 0 0 1 8-8h176a8 8 0 0 1 8 8|.2#M216 64h-40a48 48 0 0 0-96 0H40a16 16 0 0 0-16 16v120a16 16 0 0 0 16 16h176a16 16 0 0 0 16-16V80a16 16 0 0 0-16-16m-88-32a32 32 0 0 1 32 32H96a32 32 0 0 1 32-32m88 168H40V80h176Z" },
  { label: "알림", d: "M208 192H48a8 8 0 0 1-6.88-12C47.71 168.6 56 139.81 56 104a72 72 0 0 1 144 0c0 35.82 8.3 64.6 14.9 76a8 8 0 0 1-6.9 12|.2#M221.8 175.94c-5.55-9.56-13.8-36.61-13.8-71.94a80 80 0 1 0-160 0c0 35.34-8.26 62.38-13.81 71.94A16 16 0 0 0 48 200h40.81a40 40 0 0 0 78.38 0H208a16 16 0 0 0 13.8-24.06M128 216a24 24 0 0 1-22.62-16h45.24A24 24 0 0 1 128 216m-80-32c7.7-13.24 16-43.92 16-80a64 64 0 1 1 128 0c0 36.05 8.28 66.73 16 80Z" },
  { label: "프로필", d: "M192 96a64 64 0 1 1-64-64a64 64 0 0 1 64 64|.2#M230.92 212c-15.23-26.33-38.7-45.21-66.09-54.16a72 72 0 1 0-73.66 0c-27.39 8.94-50.86 27.82-66.09 54.16a8 8 0 1 0 13.85 8c18.84-32.56 52.14-52 89.07-52s70.23 19.44 89.07 52a8 8 0 1 0 13.85-8M72 96a56 56 0 1 1 56 56a56.06 56.06 0 0 1-56-56" },
];

function Icon({ spec }: { spec: string }) {
  const [a, b] = spec.split("#");
  const [pathA, opacity] = a.split("|");
  return (
    <svg className={styles.icon} viewBox="0 0 256 256" aria-hidden="true">
      <g fill="currentColor">
        <path d={pathA} opacity={opacity} />
        <path d={b} />
      </g>
    </svg>
  );
}

export default function SwipeFollow() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const trackRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const indicatorRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const tabRefs = useRef([null, null, null, null]) as unknown as Box<(HTMLButtonElement | null)[]>;
  const rafRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const track = trackRef.current;
    const indicator = indicatorRef.current;
    const tabs = tabRefs.current;
    if (!track || !indicator) return;

    let isDemo = true;
    let geo: Geo[] = [];
    let maxX = 0, indW = 0, indH = 0;
    let activeIdx = 0;
    let posX = 0;
    let vel = 0;
    let settling = false;
    let settleTarget = 0;
    let dragging = false;
    let dragStartClientX = 0;
    let dragStartX = 0;
    let lastTs = 0;

    function measure() {
      const trackRect = track!.getBoundingClientRect();
      geo = tabs.map((t) => {
        const r = t!.getBoundingClientRect();
        return { left: r.left - trackRect.left, width: r.width, height: r.height };
      });
      const first = geo[0];
      indW = first.width; indH = first.height;
      indicator!.style.width = indW + "px";
      indicator!.style.height = indH + "px";
      maxX = geo[geo.length - 1].left;
      posX = geo[activeIdx].left;
      indicator!.style.transform = "translateX(" + posX.toFixed(1) + "px)";
    }

    function human() {
      if (isDemo) { rootRef.current?.classList.remove(styles.isDemo); isDemo = false; }
    }

    function nearestIndex(centerX: number) {
      let best = 0, bestDist = Infinity;
      geo.forEach((g, i) => {
        const c = g.left + g.width / 2;
        const d = Math.abs(c - centerX);
        if (d < bestDist) { bestDist = d; best = i; }
      });
      return best;
    }

    function setActive(idx: number) {
      activeIdx = idx;
      tabs.forEach((t, i) => t?.classList.toggle(styles.isActive, i === idx));
    }

    function onPointerDown(e: PointerEvent) {
      human();
      dragging = true;
      settling = false;
      dragStartClientX = e.clientX;
      dragStartX = posX;
      if (e.pointerId != null) {
        try { track!.setPointerCapture(e.pointerId); } catch { /* noop */ }
      }
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const dx = e.clientX - dragStartClientX;
      posX = Math.max(0, Math.min(maxX, dragStartX + dx));
      indicator!.style.transform = "translateX(" + posX.toFixed(1) + "px)";
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false;
      const idx = nearestIndex(posX + indW / 2);
      setActive(idx);
      settling = true;
      settleTarget = geo[idx].left;
      vel = 0;
    }

    track.addEventListener("pointerdown", onPointerDown);
    window.addEventListener("pointermove", onPointerMove);
    window.addEventListener("pointerup", onPointerUp);
    window.addEventListener("pointercancel", onPointerUp);

    const tabHandlers = tabs.map((_, i) => () => {
      human();
      if (dragging) return;
      setActive(i);
      settling = true;
      settleTarget = geo[i].left;
      vel = 0;
    });
    tabs.forEach((t, i) => t?.addEventListener("click", tabHandlers[i]));

    const demoOrder = [2, 1, 3, 0]; // 자동 루프가 방문할 탭 순서
    let demoStep = 0;
    let demoPhase: "drag" | "settle" | "wait" = "wait";
    let demoElapsed = 0;
    let demoFromIdx = 0;

    function tick(ts: number) {
      if (!lastTs) { lastTs = ts; }
      const dt = Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;

      if (isDemo && !reduce && !dragging) {
        demoElapsed += dt * 1000;
        if (demoPhase === "drag") {
          const t = Math.min(1, demoElapsed / DRAG_MS);
          const toIdx = demoOrder[demoStep];
          const fromCenter = geo[demoFromIdx].left + indW / 2;
          const toCenter = geo[toIdx].left + indW / 2;
          const virtualCenter = fromCenter + (toCenter - fromCenter) * t;
          posX = Math.max(0, Math.min(maxX, virtualCenter - indW / 2));
          indicator!.style.transform = "translateX(" + posX.toFixed(1) + "px)";
          if (t >= 1) {
            setActive(toIdx);
            settling = true; settleTarget = geo[toIdx].left; vel = 0;
            demoPhase = "settle"; demoElapsed = 0;
          }
        } else if (demoPhase === "settle") {
          if (!settling) { demoPhase = "wait"; demoElapsed = 0; }
        } else if (demoPhase === "wait") {
          if (demoElapsed >= WAIT_MS) {
            demoFromIdx = demoOrder[demoStep];
            demoStep = (demoStep + 1) % demoOrder.length;
            demoPhase = "drag"; demoElapsed = 0;
          }
        }
      }

      if (settling) {
        const acc = (settleTarget - posX) * SPRING_K - vel * SPRING_C;
        vel += acc * dt;
        posX += vel * dt;
        if (Math.abs(settleTarget - posX) < 0.4 && Math.abs(vel) < 4) {
          posX = settleTarget; vel = 0; settling = false;
        }
        indicator!.style.transform = "translateX(" + posX.toFixed(1) + "px)";
      }

      rafRef.current = requestAnimationFrame(tick);
    }

    function onResize() { measure(); }
    measure();
    window.addEventListener("resize", onResize);
    rafRef.current = requestAnimationFrame(tick);

    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
      track.removeEventListener("pointerdown", onPointerDown);
      window.removeEventListener("pointermove", onPointerMove);
      window.removeEventListener("pointerup", onPointerUp);
      window.removeEventListener("pointercancel", onPointerUp);
      tabs.forEach((t, i) => t?.removeEventListener("click", tabHandlers[i]));
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.root + " " + styles.isDemo} ref={rootRef}>
        <div className={styles.phone}>
          <div className={styles.track} ref={trackRef}>
            <div className={styles.indicator} ref={indicatorRef} />
            {TABS.map((t, i) => (
              <button key={t.label} type="button"
                className={styles.tab + (i === 0 ? " " + styles.isActive : "")}
                aria-current={i === 0 ? "true" : undefined}
                ref={(el: HTMLButtonElement | null) => { tabRefs.current[i] = el; }}>
                <Icon spec={t.d} />
                <span className={styles.label}>{t.label}</span>
              </button>
            ))}
          </div>
        </div>
      </div>
    </main>
  );
}