01FLIP list — computing the before/after offset and releasing it into a transition

<main class="stage">
<div class="fl">
  <p class="fl__hint">The before/after coordinate delta is reversed with transform, then released so rows glide into place.</p>
  <ul class="fl__list" id="flList" role="listbox" aria-label="Playlist" aria-describedby="flHint">
    <li class="fl__row" role="option" aria-selected="false" tabindex="0"><span class="fl__rank" aria-hidden="true">1</span><span class="fl__name">Morning walk</span><span class="fl__len">3:12</span></li>
    <li class="fl__row" role="option" aria-selected="false" tabindex="0"><span class="fl__rank" aria-hidden="true">2</span><span class="fl__name">Lunch radio</span><span class="fl__len">4:05</span></li>
    <li class="fl__row" role="option" aria-selected="false" tabindex="0"><span class="fl__rank" aria-hidden="true">3</span><span class="fl__name">Deep focus</span><span class="fl__len">5:48</span></li>
    <li class="fl__row" role="option" aria-selected="false" tabindex="0"><span class="fl__rank" aria-hidden="true">4</span><span class="fl__name">Evening jazz</span><span class="fl__len">3:57</span></li>
    <li class="fl__row" role="option" aria-selected="false" tabindex="0"><span class="fl__rank" aria-hidden="true">5</span><span class="fl__name">Night sky</span><span class="fl__len">4:31</span></li>
  </ul>
  <p class="fl__meta"><b class="fl__tag" id="flState">FLIP 180ms</b><span class="fl__unit" id="flDelta">dy 0px</span></p>
</div>
</main>

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

$duration: 180ms;
$easing: cubic-bezier(.2,.8,.2,1);
$color: $subject-blue-deep;

// 파스텔 강조(도구/파스텔.py to_pastel 결과: 채도 .34 · 명도 +.11)
$p-mint: #93d9c0;

@include stage($stage-ink);

.fl {
  width: 100%;
  max-width: 380px;
  display: flex;
  flex-direction: column;
  gap: $sp-2;
}

.fl__hint {
  margin: 0;
  padding: 0 $sp-1;
  font-weight: 400;
  font-size: 11px;
  line-height: 1.4;
  color: rgba($subject-cream, .72);
}

.fl__list {
  margin: 0;
  padding: $sp-1;
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: $sp-1;
  background: $subject-cream;
  border-radius: $r-card;
  box-shadow: $shadow-press;
  overflow-y: auto;
}

.fl__row {
  display: flex;
  align-items: center;
  gap: $sp-2;
  padding: $sp-2 $sp-3;
  border-radius: $r-sm;
  background: transparent;
  color: $subject-ink;
  font-weight: 500;
  font-size: 13px;
  line-height: 1.3;
  cursor: grab;
  // will-change 없이도 transform 만 움직인다 — top/left 재계산 금지(기본규칙)
  transform: translateY(0);
}

.fl__row:hover {
  background: $p-mint;
}

.fl__rank {
  flex: 0 0 auto;
  width: 16px;
  height: 16px;
  display: grid;
  place-items: center;
  border-radius: $r-pill;
  background: $color;
  color: $subject-cream;
  font-weight: 700;
  font-size: 10px;
}

.fl__name {
  flex: 1 1 auto;
  min-width: 0;
  overflow: hidden;
  white-space: nowrap;
  text-overflow: ellipsis;
}

.fl__len {
  flex: 0 0 auto;
  font-weight: 400;
  font-size: 11px;
  color: rgba($subject-ink, .72);
}

.fl__meta {
  margin: 0;
  padding: 0 $sp-1;
  display: flex;
  align-items: baseline;
  gap: $sp-1;
}

.fl__tag {
  font-weight: 700;
  font-size: 12px;
  color: $subject-cream;
}

.fl__unit {
  font-weight: 400;
  font-size: 11px;
  color: rgba($subject-cream, .72);
}

@media (max-width: 340px) {
  .fl { max-width: 280px; gap: $sp-1; }
  .fl__hint { font-size: 10px; }
  .fl__row { font-size: 12px; padding: $sp-1 $sp-2; }
  .fl__list { max-height: 92px; gap: 0; }
}

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

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.fl {
  width: 100%;
  max-width: 380px;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.fl__hint {
  margin: 0;
  padding: 0 4px;
  font-weight: 400;
  font-size: 11px;
  line-height: 1.4;
  color: rgba(255, 247, 230, 0.72);
}

.fl__list {
  margin: 0;
  padding: 4px;
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 4px;
  background: #fff7e6;
  border-radius: 16px;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  overflow-y: auto;
}

.fl__row {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 8px 12px;
  border-radius: 8px;
  background: transparent;
  color: #17141a;
  font-weight: 500;
  font-size: 13px;
  line-height: 1.3;
  cursor: grab;
  transform: translateY(0);
}

.fl__row:hover {
  background: #93d9c0;
}

.fl__rank {
  flex: 0 0 auto;
  width: 16px;
  height: 16px;
  display: grid;
  place-items: center;
  border-radius: 999px;
  background: #0a4fe8;
  color: #fff7e6;
  font-weight: 700;
  font-size: 10px;
}

.fl__name {
  flex: 1 1 auto;
  min-width: 0;
  overflow: hidden;
  white-space: nowrap;
  text-overflow: ellipsis;
}

.fl__len {
  flex: 0 0 auto;
  font-weight: 400;
  font-size: 11px;
  color: rgba(23, 20, 26, 0.72);
}

.fl__meta {
  margin: 0;
  padding: 0 4px;
  display: flex;
  align-items: baseline;
  gap: 4px;
}

.fl__tag {
  font-weight: 700;
  font-size: 12px;
  color: #fff7e6;
}

.fl__unit {
  font-weight: 400;
  font-size: 11px;
  color: rgba(255, 247, 230, 0.72);
}

@media (max-width: 340px) {
  .fl {
    max-width: 280px;
    gap: 4px;
  }
  .fl__hint {
    font-size: 10px;
  }
  .fl__row {
    font-size: 12px;
    padding: 4px 8px;
  }
  .fl__list {
    max-height: 92px;
    gap: 0;
  }
}
@media (prefers-reduced-motion: reduce) {
  .fl__row {
    transition: none !important;
    transform: none !important;
  }
}

// FLIP 리스트(01) — 드롭으로 배열이 바뀌는 순간, 바뀌기 전 각 행의 getBoundingClientRect().top(first)을
// 재 두었다가 바뀐 뒤 좌표(last)와의 차(invert)를 transform translateY 로 즉시 되돌리고, 다음 프레임에
// transform 을 0 으로 풀어(play) 행들이 자연스럽게 미끄러지게 한다. 문구 사전과 실제 이동 픽셀은
// rAF 시계가 매 프레임 갱신한다. 포인터 조작도 같은 함수를 탄다(라이브러리 0, Pointer Events 기반).
(function () {
  var list = document.getElementById('flList');
  var stateEl = document.getElementById('flState');
  var deltaEl = document.getElementById('flDelta');
  if (!list || !stateEl || !deltaEl) return;

  var DUR = 180;          // FLIP 정리 시간(ms) — 변형 칩이 이 값만 바꾼다
  var ROWS = ['Morning walk', 'Lunch radio', 'Deep focus', 'Evening jazz', 'Night sky'];
  var LENS = ['3:12', '4:05', '5:48', '3:57', '4:31'];
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function rows() { return Array.prototype.slice.call(list.children); }

  // FLIP — 순서 배열 order 를 적용하고 이동한 행을 되돌렸다가 푼다
  function applyOrder(order) {
    var rs = rows();
    var first = rs.map(function (r) { return r.getBoundingClientRect().top; });
    // 배열 순서대로 다시 붙인다
    for (var i = 0; i < order.length; i++) list.appendChild(rs[order[i]]);
    rs = rows();
    var maxAbs = 0;
    for (var j = 0; j < rs.length; j++) {
      var dy = first[order[j]] - rs[j].getBoundingClientRect().top; // invert
      if (Math.abs(dy) > Math.abs(maxAbs)) maxAbs = dy;
      if (dy !== 0) {
        rs[j].style.transition = 'none';
        rs[j].style.transform = 'translateY(' + dy + 'px)';
      }
    }
    deltaEl.textContent = 'dy ' + Math.round(maxAbs) + 'px';
    // play — 다음 프레임에 되돌림을 푼다
    requestAnimationFrame(function () {
      rs.forEach(function (r) {
        r.style.transition = 'transform ' + DUR + 'ms cubic-bezier(.2,.8,.2,1)';
        r.style.transform = 'translateY(0)';
      });
      setTimeout(function () { rs.forEach(function (r) { r.style.transition = ''; }); }, DUR + 40);
    });
    // rank 갱신
    rs.forEach(function (r, i) { r.querySelector('.fl__rank').textContent = String(i + 1); });
    return rs;
  }

  // 사람 조작 — 행을 클릭해 아래로, Shift+클릭으로 위로 (같은 FLIP 경로)
  list.addEventListener('click', function (e) {
    stop();
    var li = e.target.closest('.fl__row');
    if (!li) return;
    var idx = rows().indexOf(li);
    var order = [0, 1, 2, 3, 4];
    if (e.shiftKey) {
      if (idx === 0) return;
      order.splice(idx - 1, 0, order.splice(idx, 1)[0]);
    } else {
      if (idx === 4) return;
      order.splice(idx + 1, 0, order.splice(idx, 1)[0]);
    }
    applyOrder(order);
  });

  // 자동 시연 — 시드 고정 시나리오: 3→1, 1→4, 원복. 한 사이클 2초.
  var playing = true;
  function stop() { playing = false; }
  var CYCLE = [
    { pick: 2, to: 0, hold: 700 },   // Deep focus 3번째 → 1번째
    { pick: 0, to: 3, hold: 700 },   // Morning walk 1번째 → 4번째
    { pick: 0, to: 1, hold: 600 }    // 1번째와 2번째 교환(원복 성분)
  ];
  function permute(pick, to) {
    var order = [0, 1, 2, 3, 4];
    order.splice(to, 0, order.splice(pick, 1)[0]);
    applyOrder(order);
  }
  if (reduce) {
    permute(2, 0);
  } else {
    var seg = 0, segStart = performance.now();
    (function step(now) {
      if (playing) {
        var s = CYCLE[seg];
        if (now - segStart >= s.hold) {
          seg = (seg + 1) % CYCLE.length;
          segStart = now;
          permute(CYCLE[seg].pick, CYCLE[seg].to);
        }
      }
      requestAnimationFrame(step);
    })(segStart);
  }
})();

// DragReorderList — vanilla 판(run/336/demo/01-drag-reorder-list/index.html)
// 의 React 포트. 배열 순서(order)를 실제 DOM 자식에 적용하기 전 각 행의 getBoundingClientRect().top
// (first)을 재 두었다가, 적용 후 좌표(last)와의 차(invert)를 transform translateY 로 즉시 되돌리고
// 다음 프레임에 transform 을 0 으로 풀어(play) 행이 미끄러지듯 정렬된다(FLIP). 사람 조작(클릭·Shift+클릭)
// 도 같은 applyOrder 를 타고, 자동 시연은 시드 고정 시나리오(CYCLE)로 같은 함수를 호출한다.
import { useEffect, useRef } from "react";
import styles from "./DragReorderList.module.scss";

export type DragReorderListProps = {
  duration?: string;
  easing?: string;
  color?: string;
};

type Box<T> = { current: T };

const HINT = "드롭 직전·직후 좌표 차를 transform 으로 되돌렸다가 풀어 행들이 미끄러져 정렬됩니다";
const ARIA_LABEL = "재생목록";

const ROW_NAMES = ["아침 산책", "점심 라디오", "오후 집중", "저녁 재즈", "밤하늘"];
const LENS = ["3:12", "4:05", "5:48", "3:57", "4:31"];

type CycleStep = { pick: number; to: number; hold: number };
const CYCLE: CycleStep[] = [
  { pick: 2, to: 0, hold: 700 },
  { pick: 0, to: 3, hold: 700 },
  { pick: 0, to: 1, hold: 600 }
];

export default function DragReorderList({
  duration = "180ms",
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#0a4fe8"
}: DragReorderListProps) {
  const listRef = useRef<HTMLUListElement | null>(null);
  const stateRef = useRef<HTMLElement | null>(null);
  const deltaRef = useRef<HTMLSpanElement | null>(null);
  const rafRef = useRef(0) as Box<number>;

  useEffect(() => {
    const list = listRef.current;
    const stateEl = stateRef.current;
    const deltaEl = deltaRef.current;
    if (!list || !stateEl || !deltaEl) return;

    const DUR = parseFloat(duration) || 180;

    function rows(): HTMLElement[] {
      return Array.prototype.slice.call(list!.children);
    }

    function applyOrder(order: number[]): HTMLElement[] {
      let rs = rows();
      const first = rs.map((r) => r.getBoundingClientRect().top);
      for (let i = 0; i < order.length; i++) list!.appendChild(rs[order[i]]);
      rs = rows();
      let maxAbs = 0;
      for (let j = 0; j < rs.length; j++) {
        const dy = first[order[j]] - rs[j].getBoundingClientRect().top;
        if (Math.abs(dy) > Math.abs(maxAbs)) maxAbs = dy;
        if (dy !== 0) {
          rs[j].style.transition = "none";
          rs[j].style.transform = "translateY(" + dy + "px)";
        }
      }
      deltaEl!.textContent = "dy " + Math.round(maxAbs) + "px";
      requestAnimationFrame(() => {
        rs.forEach((r) => {
          r.style.transition = "transform " + DUR + "ms " + easing;
          r.style.transform = "translateY(0)";
        });
        setTimeout(() => {
          rs.forEach((r) => { r.style.transition = ""; });
        }, DUR + 40);
      });
      rs.forEach((r, i) => {
        const rank = r.querySelector("." + styles.rank);
        if (rank) rank.textContent = String(i + 1);
      });
      return rs;
    }

    function stop(): void {
      playing = false;
    }

    function onClick(e: MouseEvent): void {
      stop();
      const target = e.target as HTMLElement;
      const li = target.closest("." + styles.row) as HTMLElement | null;
      if (!li) return;
      const idx = rows().indexOf(li);
      const order = [0, 1, 2, 3, 4];
      if (e.shiftKey) {
        if (idx === 0) return;
        order.splice(idx - 1, 0, order.splice(idx, 1)[0]);
      } else {
        if (idx === 4) return;
        order.splice(idx + 1, 0, order.splice(idx, 1)[0]);
      }
      applyOrder(order);
    }
    list.addEventListener("click", onClick as EventListener);

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let playing = true;
    function permute(pick: number, to: number): void {
      const order = [0, 1, 2, 3, 4];
      order.splice(to, 0, order.splice(pick, 1)[0]);
      applyOrder(order);
    }

    if (reduce) {
      permute(2, 0);
    } else {
      let seg = 0;
      let segStart = performance.now();
      const step = (now: number): void => {
        if (playing) {
          const s = CYCLE[seg];
          if (now - segStart >= s.hold) {
            seg = (seg + 1) % CYCLE.length;
            segStart = now;
            permute(CYCLE[seg].pick, CYCLE[seg].to);
          }
        }
        rafRef.current = requestAnimationFrame(step);
      };
      rafRef.current = requestAnimationFrame(step);
    }

    return () => {
      cancelAnimationFrame(rafRef.current);
      list.removeEventListener("click", onClick as EventListener);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.fl}>
        <p className={styles.hint} id="flHintReact">{HINT}</p>
        <ul className={styles.list} ref={listRef} role="listbox" aria-label={ARIA_LABEL} aria-describedby="flHintReact">
          {ROW_NAMES.map((name, i) => (
            <li key={i} className={styles.row} role="option" aria-selected="false" tabIndex={0}>
              <span className={styles.rank} aria-hidden="true" style={{ background: color }}>{i + 1}</span>
              <span className={styles.name}>{name}</span>
              <span className={styles.len}>{LENS[i]}</span>
            </li>
          ))}
        </ul>
        <p className={styles.meta}>
          <b className={styles.tag} ref={stateRef}>FLIP {duration}</b>
          <span className={styles.unit} ref={deltaRef}>dy 0px</span>
        </p>
      </div>
    </main>
  );
}