01수량을 바꾸면 소계와 합계가 함께 물드는 줄

<main class="stage">
  <section class="qs is-demo" id="qs" aria-label="장바구니 수량과 합계">
    <header class="qs__head">
      <p class="qs__title">장바구니</p>
      <p class="qs__hint">수량을 눌러 보세요</p>
    </header>
    <ul class="qs__list">
      <li class="qs__row" data-qty="2">
        <span class="qs__info">
          <span class="qs__name">기본 티셔츠</span>
          <span class="qs__unit">12,000원</span>
        </span>
        <span class="qs__step">
          <button class="qs__b qs__b--down" type="button" data-step="-1" aria-label="기본 티셔츠 수량 줄이기"><svg viewBox="0 0 256 256" width="12" height="12" fill="currentColor" aria-hidden="true"><path d="M216,56V200a16,16,0,0,1-16,16H56a16,16,0,0,1-16-16V56A16,16,0,0,1,56,40H200A16,16,0,0,1,216,56Z" opacity="0.2"/><path d="M224,128a8,8,0,0,1-8,8H40a8,8,0,0,1,0-16H216A8,8,0,0,1,224,128Z"/></svg></button>
          <output class="qs__q qs__swap"><span class="qs__v">2</span><span class="qs__alt" aria-hidden="true">3</span></output>
          <button class="qs__b qs__b--up" type="button" data-step="1" aria-label="기본 티셔츠 수량 늘리기"><svg viewBox="0 0 256 256" width="12" height="12" fill="currentColor" aria-hidden="true"><path d="M216,56V200a16,16,0,0,1-16,16H56a16,16,0,0,1-16-16V56A16,16,0,0,1,56,40H200A16,16,0,0,1,216,56Z" opacity="0.2"/><path d="M224,128a8,8,0,0,1-8,8H136v80a8,8,0,0,1-16,0V136H40a8,8,0,0,1,0-16h80V40a8,8,0,0,1,16,0v80h80A8,8,0,0,1,224,128Z"/></svg></button>
        </span>
        <span class="qs__sum qs__swap"><span class="qs__v qs__money">24,000원</span><span class="qs__alt" aria-hidden="true">36,000원</span></span>
      </li>
      <li class="qs__row" data-qty="1">
        <span class="qs__info">
          <span class="qs__name">무지 양말</span>
          <span class="qs__unit">4,000원</span>
        </span>
        <span class="qs__step">
          <button class="qs__b qs__b--down" type="button" data-step="-1" aria-label="무지 양말 수량 줄이기"><svg viewBox="0 0 256 256" width="12" height="12" fill="currentColor" aria-hidden="true"><path d="M216,56V200a16,16,0,0,1-16,16H56a16,16,0,0,1-16-16V56A16,16,0,0,1,56,40H200A16,16,0,0,1,216,56Z" opacity="0.2"/><path d="M224,128a8,8,0,0,1-8,8H40a8,8,0,0,1,0-16H216A8,8,0,0,1,224,128Z"/></svg></button>
          <output class="qs__q"><span class="qs__v">1</span></output>
          <button class="qs__b qs__b--up" type="button" data-step="1" aria-label="무지 양말 수량 늘리기"><svg viewBox="0 0 256 256" width="12" height="12" fill="currentColor" aria-hidden="true"><path d="M216,56V200a16,16,0,0,1-16,16H56a16,16,0,0,1-16-16V56A16,16,0,0,1,56,40H200A16,16,0,0,1,216,56Z" opacity="0.2"/><path d="M224,128a8,8,0,0,1-8,8H136v80a8,8,0,0,1-16,0V136H40a8,8,0,0,1,0-16h80V40a8,8,0,0,1,16,0v80h80A8,8,0,0,1,224,128Z"/></svg></button>
        </span>
        <span class="qs__sum"><span class="qs__v qs__money">4,000원</span></span>
      </li>
    </ul>
    <p class="qs__total">
      <span class="qs__tlabel">합계</span>
      <output class="qs__sum qs__sum--grand qs__swap" id="qs-grand"><span class="qs__v" id="qs-grandv">28,000원</span><span class="qs__alt" aria-hidden="true">40,000원</span></output>
    </p>
  </section>
</main>

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

// 수량을 바꾸면 그 줄 소계와 아래 합계가 같은 순간에 다시 계산되고,
// 바뀐 숫자 칸 두 곳만 background-color 가 켜졌다 transition 으로 식는다.
// 숫자는 font-variant-numeric: tabular-nums 라 자릿수가 늘어도 칸이 흔들리지 않는다.
$duration: $dur-loop;
$easing: $ease-spring;
$color: $stage-yellow;

@include stage($stage-ink);

.qs {
  box-sizing: border-box;
  width: min(300px, 92vw);
  padding: $sp-3;
  border-radius: $r-card;
  background: $subject-cream;
  color: $subject-ink;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}
.qs * { box-sizing: border-box; }

.qs__head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: $sp-2;
}
.qs__title {
  margin: 0;
  font-size: 13px;
  font-weight: 700;
  line-height: 1.3;
}
.qs__hint {
  margin: 0;
  font-size: 10px;
  font-weight: 500;
  line-height: 1.3;
  color: rgba($subject-ink, .72);
}

.qs__list {
  display: grid;
  gap: $sp-2;
  margin: $sp-2 0 0;
  padding: 0;
  list-style: none;
}
.qs__row {
  display: grid;
  grid-template-columns: 1fr auto auto;
  align-items: center;
  gap: $sp-2;
}
.qs__info {
  display: grid;
  min-width: 0;
}
.qs__name {
  font-size: 11px;
  font-weight: 700;
  line-height: 1.35;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}
.qs__unit {
  font-size: 10px;
  font-weight: 500;
  line-height: 1.35;
  color: rgba($subject-ink, .72);
  font-variant-numeric: tabular-nums;
}

.qs__step {
  display: inline-flex;
  align-items: center;
  gap: $sp-1;
  padding: $sp-1;
  border-radius: $r-pill;
  background: rgba(23, 20, 26, .07);
}
.qs__b {
  display: inline-grid;
  place-items: center;
  width: 20px;
  height: 20px;
  padding: 0;
  margin: 0;
  border: 0;
  border-radius: 50%;
  background: $subject-cream;
  color: $subject-ink;
  cursor: pointer;
  &:focus-visible { outline: 2px solid $subject-blue; outline-offset: 2px; }
}

// 숫자 칸 — 자릿수가 바뀌어도 폭이 흔들리지 않게 tabular-nums + min-width
.qs__q {
  position: relative;
  min-width: 18px;
  text-align: center;
  font-size: 12px;
  font-weight: 700;
  line-height: 1.4;
  font-variant-numeric: tabular-nums;
}
.qs__sum {
  position: relative;
  justify-self: end;
  min-width: 58px;
  padding: $sp-1;
  border-radius: $r-xs;
  text-align: right;
  font-size: 11px;
  font-weight: 700;
  line-height: 1.4;
  font-variant-numeric: tabular-nums;
  // 실제 클릭 경로: 배경을 한 번 켜고(transition: none) 클래스를 떼면 이 transition 이 식힌다
  background-color: rgba($stage-yellow, 0);
  transition: background-color $dur-enter $ease-out;
  &.is-hot {
    background-color: rgba($stage-yellow, .62);
    transition: none;
  }
}
// 미리보기 루프에서만 쓰는 두 번째 숫자 — 정적 경로에서는 자리를 차지하지 않고 보이지도 않는다
.qs__alt {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center;
  opacity: 0;
}
.qs__sum .qs__alt {
  place-items: center end;
  padding: $sp-1;
}

.qs__total {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: $sp-2;
  margin: $sp-2 0 0;
  padding: $sp-2 0 0;
  border-top: 1px solid rgba(23, 20, 26, .14);
}
.qs__tlabel {
  font-size: 11px;
  font-weight: 700;
  line-height: 1.4;
}
.qs__sum--grand {
  min-width: 68px;
  font-size: 13px;
}

// ── 격자 미리보기 루프 (2초) ──
// 숫자 교체는 steps(1, end) 컷이다 — 크로스페이드로 겹치면 대표 이미지에 두 숫자가 반투명하게 겹친다.
// 컷만으로는 움직인 프레임이 모자라니 칸이 살짝 솟았다 내려오는 translateY 를 곁들인다.
@keyframes qs-a {
  0%, 24% { opacity: 1; }
  25%, 62% { opacity: 0; }
  63%, 100% { opacity: 1; }
}
@keyframes qs-b {
  0%, 24% { opacity: 0; }
  25%, 62% { opacity: 1; }
  63%, 100% { opacity: 0; }
}
@keyframes qs-tint {
  0%, 24% { background-color: rgba($stage-yellow, 0); }
  25% { background-color: rgba($stage-yellow, .62); }
  54% { background-color: rgba($stage-yellow, 0); }
  62% { background-color: rgba($stage-yellow, 0); }
  63% { background-color: rgba($stage-yellow, .62); }
  88%, 100% { background-color: rgba($stage-yellow, 0); }
}
@keyframes qs-pop {
  0%, 24% { transform: translateY(0); }
  25% { transform: translateY(-4px); }
  40% { transform: translateY(0); }
  62% { transform: translateY(0); }
  63% { transform: translateY(-4px); }
  78%, 100% { transform: translateY(0); }
}
@keyframes qs-press-up {
  0%, 21% { transform: scale(1); }
  25% { transform: scale(.78); }
  37%, 100% { transform: scale(1); }
}
@keyframes qs-press-down {
  0%, 59% { transform: scale(1); }
  63% { transform: scale(.78); }
  75%, 100% { transform: scale(1); }
}

.qs.is-demo {
  .qs__swap .qs__v { animation-name: qs-a; }
  .qs__swap .qs__alt { animation-name: qs-b; }
  .qs__swap .qs__v,
  .qs__swap .qs__alt {
    animation-duration: $duration;
    animation-timing-function: steps(1, end);
    animation-iteration-count: infinite;
    animation-fill-mode: backwards;
  }
  .qs__sum.qs__swap {
    animation-name: qs-tint, qs-pop;
    animation-duration: $duration, $duration;
    animation-timing-function: $ease-out, $easing;
    animation-iteration-count: infinite, infinite;
    animation-fill-mode: backwards, backwards;
  }
  .qs__row:first-child .qs__b--up { animation-name: qs-press-up; }
  .qs__row:first-child .qs__b--down { animation-name: qs-press-down; }
  .qs__row:first-child .qs__b {
    animation-duration: $duration;
    animation-timing-function: $easing;
    animation-iteration-count: infinite;
    animation-fill-mode: backwards;
  }
}

@media (max-width: 340px) {
  .qs { padding: $sp-2; }
  .qs__sum { min-width: 52px; }
  .qs__sum--grand { min-width: 60px; }
  .qs__b { width: 18px; height: 18px; }
}

@media (prefers-reduced-motion: reduce) {
  .qs.is-demo .qs__v,
  .qs.is-demo .qs__b,
  .qs.is-demo .qs__sum {
    animation: none !important;
    transform: none !important;
  }
  .qs.is-demo .qs__v { opacity: 1 !important; }
  // 정지 상태에서도 값은 보인다 — 겹쳐 둔 두 번째 숫자만 계속 감춘다
  .qs.is-demo .qs__alt { animation: none !important; opacity: 0 !important; }
  .qs.is-demo .qs__sum { background-color: rgba($stage-yellow, 0) !important; }
  .qs__sum { 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: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.qs {
  box-sizing: border-box;
  width: min(300px, 92vw);
  padding: 12px;
  border-radius: 16px;
  background: #fff7e6;
  color: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.qs * {
  box-sizing: border-box;
}

.qs__head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 8px;
}

.qs__title {
  margin: 0;
  font-size: 13px;
  font-weight: 700;
  line-height: 1.3;
}

.qs__hint {
  margin: 0;
  font-size: 10px;
  font-weight: 500;
  line-height: 1.3;
  color: rgba(23, 20, 26, 0.72);
}

.qs__list {
  display: grid;
  gap: 8px;
  margin: 8px 0 0;
  padding: 0;
  list-style: none;
}

.qs__row {
  display: grid;
  grid-template-columns: 1fr auto auto;
  align-items: center;
  gap: 8px;
}

.qs__info {
  display: grid;
  min-width: 0;
}

.qs__name {
  font-size: 11px;
  font-weight: 700;
  line-height: 1.35;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.qs__unit {
  font-size: 10px;
  font-weight: 500;
  line-height: 1.35;
  color: rgba(23, 20, 26, 0.72);
  font-variant-numeric: tabular-nums;
}

.qs__step {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  padding: 4px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.07);
}

.qs__b {
  display: inline-grid;
  place-items: center;
  width: 20px;
  height: 20px;
  padding: 0;
  margin: 0;
  border: 0;
  border-radius: 50%;
  background: #fff7e6;
  color: #17141a;
  cursor: pointer;
}
.qs__b:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

.qs__q {
  position: relative;
  min-width: 18px;
  text-align: center;
  font-size: 12px;
  font-weight: 700;
  line-height: 1.4;
  font-variant-numeric: tabular-nums;
}

.qs__sum {
  position: relative;
  justify-self: end;
  min-width: 58px;
  padding: 4px;
  border-radius: 4px;
  text-align: right;
  font-size: 11px;
  font-weight: 700;
  line-height: 1.4;
  font-variant-numeric: tabular-nums;
  background-color: rgba(255, 210, 63, 0);
  transition: background-color 600ms ease-out;
}
.qs__sum.is-hot {
  background-color: rgba(255, 210, 63, 0.62);
  transition: none;
}

.qs__alt {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center;
  opacity: 0;
}

.qs__sum .qs__alt {
  place-items: center end;
  padding: 4px;
}

.qs__total {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  margin: 8px 0 0;
  padding: 8px 0 0;
  border-top: 1px solid rgba(23, 20, 26, 0.14);
}

.qs__tlabel {
  font-size: 11px;
  font-weight: 700;
  line-height: 1.4;
}

.qs__sum--grand {
  min-width: 68px;
  font-size: 13px;
}

@keyframes qs-a {
  0%, 24% {
    opacity: 1;
  }
  25%, 62% {
    opacity: 0;
  }
  63%, 100% {
    opacity: 1;
  }
}
@keyframes qs-b {
  0%, 24% {
    opacity: 0;
  }
  25%, 62% {
    opacity: 1;
  }
  63%, 100% {
    opacity: 0;
  }
}
@keyframes qs-tint {
  0%, 24% {
    background-color: rgba(255, 210, 63, 0);
  }
  25% {
    background-color: rgba(255, 210, 63, 0.62);
  }
  54% {
    background-color: rgba(255, 210, 63, 0);
  }
  62% {
    background-color: rgba(255, 210, 63, 0);
  }
  63% {
    background-color: rgba(255, 210, 63, 0.62);
  }
  88%, 100% {
    background-color: rgba(255, 210, 63, 0);
  }
}
@keyframes qs-pop {
  0%, 24% {
    transform: translateY(0);
  }
  25% {
    transform: translateY(-4px);
  }
  40% {
    transform: translateY(0);
  }
  62% {
    transform: translateY(0);
  }
  63% {
    transform: translateY(-4px);
  }
  78%, 100% {
    transform: translateY(0);
  }
}
@keyframes qs-press-up {
  0%, 21% {
    transform: scale(1);
  }
  25% {
    transform: scale(0.78);
  }
  37%, 100% {
    transform: scale(1);
  }
}
@keyframes qs-press-down {
  0%, 59% {
    transform: scale(1);
  }
  63% {
    transform: scale(0.78);
  }
  75%, 100% {
    transform: scale(1);
  }
}
.qs.is-demo .qs__swap .qs__v {
  animation-name: qs-a;
}
.qs.is-demo .qs__swap .qs__alt {
  animation-name: qs-b;
}
.qs.is-demo .qs__swap .qs__v,
.qs.is-demo .qs__swap .qs__alt {
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
  animation-fill-mode: backwards;
}
.qs.is-demo .qs__sum.qs__swap {
  animation-name: qs-tint, qs-pop;
  animation-duration: 2s, 2s;
  animation-timing-function: ease-out, cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite, infinite;
  animation-fill-mode: backwards, backwards;
}
.qs.is-demo .qs__row:first-child .qs__b--up {
  animation-name: qs-press-up;
}
.qs.is-demo .qs__row:first-child .qs__b--down {
  animation-name: qs-press-down;
}
.qs.is-demo .qs__row:first-child .qs__b {
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite;
  animation-fill-mode: backwards;
}

@media (max-width: 340px) {
  .qs {
    padding: 8px;
  }
  .qs__sum {
    min-width: 52px;
  }
  .qs__sum--grand {
    min-width: 60px;
  }
  .qs__b {
    width: 18px;
    height: 18px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .qs.is-demo .qs__v,
  .qs.is-demo .qs__b,
  .qs.is-demo .qs__sum {
    animation: none !important;
    transform: none !important;
  }
  .qs.is-demo .qs__v {
    opacity: 1 !important;
  }
  .qs.is-demo .qs__alt {
    animation: none !important;
    opacity: 0 !important;
  }
  .qs.is-demo .qs__sum {
    background-color: rgba(255, 210, 63, 0) !important;
  }
  .qs__sum {
    transition: none !important;
  }
}

// 수량을 바꾸면 그 줄 소계와 아래 합계를 같은 순간에 다시 계산한다.
// 금액 서식은 화면에 이미 있는 단가 글자에서 뽑는다 — 언어팩이 바꿔 둔 통화 표기를 그대로 따라간다
var qsRoot = document.getElementById('qs');
var qsRows = qsRoot.querySelectorAll('.qs__row');
var qsGrand = document.getElementById('qs-grand');
var qsGrandValue = document.getElementById('qs-grandv');

function qsComma(n) { return String(n).replace(/\B(?=(\d{3})+$)/g, ','); }
function qsUnitText(row) { return row.querySelector('.qs__unit').textContent.trim(); }
function qsAmount(text) { return Number(text.replace(/[^\d]/g, '')); }
function qsMoney(row, n) { return qsUnitText(row).replace(/[\d,]+/, qsComma(n)); }

function qsPaint(row) {
  var qty = Number(row.getAttribute('data-qty'));
  row.querySelector('.qs__q .qs__v').textContent = String(qty);
  row.querySelector('.qs__money').textContent = qsMoney(row, qty * qsAmount(qsUnitText(row)));
}
function qsTotal() {
  var sum = 0;
  for (var i = 0; i < qsRows.length; i++) {
    sum += Number(qsRows[i].getAttribute('data-qty')) * qsAmount(qsUnitText(qsRows[i]));
  }
  qsGrandValue.textContent = qsMoney(qsRows[0], sum);
}
// 물들었다 식는다 — 배경만 한 번 켜고 transition 이 되돌린다
function qsTint(cell) {
  cell.classList.add('is-hot');
  setTimeout(function () { cell.classList.remove('is-hot'); }, 80);
}
function qsBump(row, step) {
  qsRoot.classList.remove('is-demo');
  var qty = Math.min(9, Math.max(1, Number(row.getAttribute('data-qty')) + step));
  row.setAttribute('data-qty', String(qty));
  qsPaint(row);
  qsTotal();
  qsTint(row.querySelector('.qs__sum'));
  qsTint(qsGrand);
}
qsRoot.addEventListener('click', function (e) {
  var btn = e.target.closest('.qs__b');
  if (btn) { qsBump(btn.closest('.qs__row'), Number(btn.getAttribute('data-step'))); }
});

import { useState } from "react";
import type { CSSProperties } from "react";
import styles from "./CartQtySubtotalSync.module.scss";

export type CartQtyLine = {
  id: string;
  name: string;
  /** 단가 글자 그대로 — 금액 서식(통화 기호·자릿점)을 여기서 뽑아 쓴다 */
  unit: string;
  qty: number;
  downLabel: string;
  upLabel: string;
};

export type CartQtySubtotalSyncProps = {
  title?: string;
  hint?: string;
  totalLabel?: string;
  groupLabel?: string;
  lines?: CartQtyLine[];
  /** 격자 미리보기용 2초 자동 루프. 사람이 손대면 꺼진다 */
  demo?: boolean;
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };

const DEFAULT_LINES: CartQtyLine[] = [
  { id: "tee", name: "기본 티셔츠", unit: "12,000원", qty: 2, downLabel: "기본 티셔츠 수량 줄이기", upLabel: "기본 티셔츠 수량 늘리기" },
  { id: "socks", name: "무지 양말", unit: "4,000원", qty: 1, downLabel: "무지 양말 수량 줄이기", upLabel: "무지 양말 수량 늘리기" },
];

function comma(n: number): string {
  return String(n).replace(/\B(?=(\d{3})+$)/g, ",");
}
function amount(text: string): number {
  return Number(text.replace(/[^\d]/g, ""));
}
// 금액 서식은 단가 글자를 그대로 본떠 만든다 — 언어팩이 바꿔 둔 통화 표기를 따라간다
function money(unit: string, n: number): string {
  return unit.replace(/[\d,]+/, comma(n));
}

function MinusIcon() {
  return (
    <svg viewBox="0 0 256 256" width="12" height="12" fill="currentColor" aria-hidden="true"><path d="M216,56V200a16,16,0,0,1-16,16H56a16,16,0,0,1-16-16V56A16,16,0,0,1,56,40H200A16,16,0,0,1,216,56Z" opacity="0.2"/><path d="M224,128a8,8,0,0,1-8,8H40a8,8,0,0,1,0-16H216A8,8,0,0,1,224,128Z"/></svg>
  );
}
function PlusIcon() {
  return (
    <svg viewBox="0 0 256 256" width="12" height="12" fill="currentColor" aria-hidden="true"><path d="M216,56V200a16,16,0,0,1-16,16H56a16,16,0,0,1-16-16V56A16,16,0,0,1,56,40H200A16,16,0,0,1,216,56Z" opacity="0.2"/><path d="M224,128a8,8,0,0,1-8,8H136v80a8,8,0,0,1-16,0V136H40a8,8,0,0,1,0-16h80V40a8,8,0,0,1,16,0v80h80A8,8,0,0,1,224,128Z"/></svg>
  );
}

export default function CartQtySubtotalSync({
  title = "장바구니",
  hint = "수량을 눌러 보세요",
  totalLabel = "합계",
  groupLabel = "장바구니 수량과 합계",
  lines = DEFAULT_LINES,
  demo = true,
  duration = 2000,
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#ffd23f",
}: CartQtySubtotalSyncProps) {
  const [qty, setQty] = useState<number[]>(lines.map((l) => l.qty));
  const [hot, setHot] = useState<string[]>([]);
  const [live, setLive] = useState(demo);

  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };
  const grand = lines.reduce((sum, l, i) => sum + qty[i] * amount(l.unit), 0);

  // 미리보기 루프가 컷으로 갈아 끼우는 두 번째 값 — 첫 줄 수량이 하나 늘었을 때의 숫자
  const altQty = lines[0].qty + 1;
  const altGrand = lines.reduce((sum, l, i) => sum + (i === 0 ? altQty : l.qty) * amount(l.unit), 0);

  // 수량·소계·합계를 같은 순간에 다시 계산하고, 바뀐 숫자 칸 두 곳만 배경을 한 번 켠다
  function bump(index: number, step: number) {
    setLive(false);
    setQty((prev: number[]) => prev.map((q, k) => (k === index ? Math.min(9, Math.max(1, q + step)) : q)));
    setHot([lines[index].id, "grand"]);
    setTimeout(() => setHot([]), 80);
  }

  const cls = [styles.qs, live ? styles.isDemo : ""].filter(Boolean).join(" ");
  const grandCls = [styles.sum, styles.grand, styles.swap, hot.indexOf("grand") >= 0 ? styles.isHot : ""].filter(Boolean).join(" ");

  return (
    <section className={cls} style={vars} aria-label={groupLabel}>
      <header className={styles.head}>
        <p className={styles.title}>{title}</p>
        <p className={styles.hint}>{hint}</p>
      </header>
      <ul className={styles.list}>
        {lines.map((line, i) => {
          const swap = i === 0;
          const sumCls = [styles.sum, swap ? styles.swap : "", hot.indexOf(line.id) >= 0 ? styles.isHot : ""].filter(Boolean).join(" ");
          return (
            <li className={styles.row} key={line.id}>
              <span className={styles.info}>
                <span className={styles.name}>{line.name}</span>
                <span className={styles.unit}>{line.unit}</span>
              </span>
              <span className={styles.step}>
                <button className={`${styles.btn} ${styles.btnDown}`} type="button" onClick={() => bump(i, -1)} aria-label={line.downLabel}><MinusIcon /></button>
                <output className={`${styles.qty} ${swap ? styles.swap : ""}`}>
                  <span className={styles.value}>{qty[i]}</span>
                  {swap && live ? <span className={styles.alt} aria-hidden="true">{altQty}</span> : null}
                </output>
                <button className={`${styles.btn} ${styles.btnUp}`} type="button" onClick={() => bump(i, 1)} aria-label={line.upLabel}><PlusIcon /></button>
              </span>
              <span className={sumCls}>
                <span className={styles.value}>{money(line.unit, qty[i] * amount(line.unit))}</span>
                {swap && live ? <span className={styles.alt} aria-hidden="true">{money(line.unit, altQty * amount(line.unit))}</span> : null}
              </span>
            </li>
          );
        })}
      </ul>
      <p className={styles.total}>
        <span className={styles.tlabel}>{totalLabel}</span>
        <output className={grandCls}>
          <span className={styles.value}>{money(lines[0].unit, grand)}</span>
          {live ? <span className={styles.alt} aria-hidden="true">{money(lines[0].unit, altGrand)}</span> : null}
        </output>
      </p>
    </section>
  );
}