05見出しだけ先に変え本文が後から追う楽観的な遷移

<main class="stage">
  <section class="oh is-demo" id="oh" data-tab="1" aria-label="楽観的な画面移動">
    <h2 class="oh__title">
      <span class="oh__t oh__t--1">今週のまとめ</span>
      <span class="oh__t oh__t--2">最近の動き</span>
      <span class="oh__t oh__t--3">通知の設定</span>
    </h2>
    <div class="oh__tabs" role="tablist" id="oh-tabs" aria-label="画面を選ぶ">
      <button class="oh__tab oh__tab--1" type="button" role="tab" id="oh-tab-1" aria-controls="oh-body" aria-selected="true">
        <span class="oh__pill" aria-hidden="true"></span><span class="oh__lab">まとめ</span>
      </button>
      <button class="oh__tab oh__tab--2" type="button" role="tab" id="oh-tab-2" aria-controls="oh-body" aria-selected="false">
        <span class="oh__pill" aria-hidden="true"></span><span class="oh__lab">動き</span>
      </button>
      <button class="oh__tab oh__tab--3" type="button" role="tab" id="oh-tab-3" aria-controls="oh-body" aria-selected="false">
        <span class="oh__pill" aria-hidden="true"></span><span class="oh__lab">設定</span>
      </button>
    </div>
    <div class="oh__body" id="oh-body" role="tabpanel" aria-labelledby="oh-tab-1" aria-busy="false">
      <div class="oh__panel oh__panel--1">
        <p class="oh__lead">訪問 1,284人</p>
        <p class="oh__sub">先週より12%増えました</p>
      </div>
      <div class="oh__panel oh__panel--2">
        <p class="oh__lead">新しいコメント7件</p>
        <p class="oh__sub">まだ返事していない投稿が2件</p>
      </div>
      <div class="oh__panel oh__panel--3">
        <p class="oh__lead">通知オン</p>
        <p class="oh__sub">週間まとめメールを受信中</p>
      </div>
      <div class="oh__sk" aria-hidden="true">
        <span class="oh__sk-bar"><span class="oh__shine"></span></span>
        <span class="oh__sk-bar"><span class="oh__shine"></span></span>
        <span class="oh__sk-bar"><span class="oh__shine"></span></span>
      </div>
    </div>
  </section>
</main>

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

// 낙관적 이동 — 탭을 누른 그 순간 제목이 steps(1, end) 로 컷 교체되고,
// 본문은 아직 오지 않아 aria-busy 를 켠 채 자리만 지키다 translateY 로 뒤늦게 올라온다.
// 제목 두 장은 grid-area 로 겹쳐 두고 컷으로만 갈아 끼운다 — opacity 크로스페이드는 반투명 유령을 만든다.
// 본문 자리표시(스켈레톤)는 교체가 시작되는 그 프레임에 켜서, 중간 프레임이 빈 구멍이 되지 않게 한다.
$duration: $dur-base;
$easing: $ease-spring;
$color: $subject-cream;

@include stage($stage-ink);

$line: rgba(255, 247, 230, .18);

.oh {
  box-sizing: border-box;
  display: grid;
  gap: $sp-3;
  width: min(300px, 94vw);
  color: $color;
}
.oh * { box-sizing: border-box; }

// ── 헤더 제목: 세 장을 같은 칸에 겹쳐 두고 컷으로 바꾼다 ──
.oh__title {
  display: grid;
  margin: 0;
  font-size: 19px;
  font-weight: 800;
  line-height: 1.2;
}
.oh__t {
  grid-area: 1 / 1;
  opacity: 0;
}
.oh[data-tab="1"] .oh__t--1,
.oh[data-tab="2"] .oh__t--2,
.oh[data-tab="3"] .oh__t--3 { opacity: 1; }

// ── 탭 줄: 1fr 세 칸. 인디케이터를 고정 px 로 옮기지 않고 탭마다 제 알약을 켠다 ──
.oh__tabs {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: $sp-1;
}
.oh__tab {
  position: relative;
  margin: 0;
  padding: $sp-2 0;
  border: 0;
  border-radius: $r-pill;
  background: none;
  color: inherit;
  font-family: inherit;
  font-size: 13px;
  font-weight: 700;
  line-height: 1;
  cursor: pointer;
}
.oh__tab:focus-visible { outline: 2px solid $color; outline-offset: 2px; }
.oh__pill {
  position: absolute;
  inset: 0;
  border-radius: $r-pill;
  background: $color;
  opacity: 0;
}
.oh__lab { position: relative; }
.oh[data-tab="1"] .oh__tab--1 .oh__pill,
.oh[data-tab="2"] .oh__tab--2 .oh__pill,
.oh[data-tab="3"] .oh__tab--3 .oh__pill { opacity: .16; }

// ── 본문: 패널 셋과 자리표시가 같은 칸에 겹친다 ──
.oh__body {
  display: grid;
  height: 96px;
  padding: $sp-3;
  border: 1px solid $line;
  border-radius: $r-card;
}
.oh__panel,
.oh__sk { grid-area: 1 / 1; }

.oh__panel {
  display: grid;
  align-content: start;
  gap: $sp-2;
  opacity: 0;
  transform: translateY(10px);
  transition: transform $duration $easing, opacity $dur-instant linear;
}
.oh[data-tab="1"] .oh__panel--1,
.oh[data-tab="2"] .oh__panel--2,
.oh[data-tab="3"] .oh__panel--3 { opacity: 1; transform: translateY(0); }
// 아직 안 온 본문 — 위 규칙과 같은 특이도라 뒤에 오는 이 규칙이 이긴다
.oh.is-loading .oh__panel { opacity: 0; transform: translateY(10px); }

.oh__lead {
  margin: 0;
  font-size: 17px;
  font-weight: 800;
  line-height: 1.3;
}
.oh__sub {
  margin: 0;
  color: rgba(255, 247, 230, .66);
  font-size: 12px;
  font-weight: 700;
  line-height: 1.4;
}

// ── 자리표시: 본문이 오기 전까지 같은 크기의 자리를 지킨다 ──
.oh__sk {
  display: grid;
  align-content: start;
  gap: $sp-2;
  opacity: 0;
}
.oh.is-loading .oh__sk { opacity: 1; }
.oh__sk-bar {
  position: relative;
  overflow: hidden;
  height: 12px;
  border-radius: $r-pill;
  background: rgba(255, 247, 230, .14);
}
.oh__sk-bar:nth-child(1) { width: 64%; }
.oh__sk-bar:nth-child(2) { width: 92%; }
.oh__sk-bar:nth-child(3) { width: 48%; }
.oh__shine {
  position: absolute;
  inset: 0;
  background: linear-gradient(90deg, rgba(255, 247, 230, 0), rgba(255, 247, 230, .4), rgba(255, 247, 230, 0));
  transform: translateX(-100%);
}

// ── 격자 미리보기 전용 2초 루프 ──
// 10% 에 탭 1 → 탭 2 로 넘어가고(제목·알약은 컷), 자리표시는 같은 프레임에 켜진다.
// 40% 에 본문이 도착해 컷으로 나타나며 translateY 로 올라오고, 86% 에 한 프레임 컷으로 처음 상태로 돌아온다.
@keyframes oh-a-on {
  0%, 9% { opacity: 1; }
  10%, 85% { opacity: 0; }
  86%, 100% { opacity: 1; }
}
// 제목은 누른 그 프레임(10%)에 바뀐다 — 낙관적이라는 말의 뜻이 여기 있다
@keyframes oh-b-now {
  0%, 9% { opacity: 0; }
  10%, 85% { opacity: 1; }
  86%, 100% { opacity: 0; }
}
// 본문은 0.6초쯤 늦은 40% 에야 도착한다
@keyframes oh-b-on {
  0%, 39% { opacity: 0; }
  40%, 85% { opacity: 1; }
  86%, 100% { opacity: 0; }
}
@keyframes oh-pill-a {
  0%, 9% { opacity: .16; }
  10%, 85% { opacity: 0; }
  86%, 100% { opacity: .16; }
}
@keyframes oh-pill-b {
  0%, 9% { opacity: 0; }
  10%, 85% { opacity: .16; }
  86%, 100% { opacity: 0; }
}
@keyframes oh-sk-on {
  0%, 9% { opacity: 0; }
  10%, 39% { opacity: 1; }
  40%, 100% { opacity: 0; }
}
// 컷만 있으면 움직인 프레임이 모자라니 transform 기반 움직임을 옆에 붙인다
@keyframes oh-a-rise {
  0%, 85% { transform: translateY(0); }
  86% { transform: translateY(10px); }
  97%, 100% { transform: translateY(0); }
}
@keyframes oh-b-rise {
  0%, 40% { transform: translateY(10px); }
  64%, 85% { transform: translateY(0); }
  86%, 100% { transform: translateY(10px); }
}
@keyframes oh-shine {
  0% { transform: translateX(-100%); }
  46% { transform: translateX(220%); }
  47%, 100% { transform: translateX(-100%); }
}

.oh.is-demo {
  .oh__t--1 {
    animation-name: oh-a-on;
    animation-duration: $dur-loop;
    animation-timing-function: steps(1, end);
    animation-iteration-count: infinite;
  }
  .oh__t--2 {
    animation-name: oh-b-now;
    animation-duration: $dur-loop;
    animation-timing-function: steps(1, end);
    animation-iteration-count: infinite;
  }
  .oh__tab--1 .oh__pill {
    animation-name: oh-pill-a;
    animation-duration: $dur-loop;
    animation-timing-function: steps(1, end);
    animation-iteration-count: infinite;
  }
  .oh__tab--2 .oh__pill {
    animation-name: oh-pill-b;
    animation-duration: $dur-loop;
    animation-timing-function: steps(1, end);
    animation-iteration-count: infinite;
  }
  .oh__sk {
    animation-name: oh-sk-on;
    animation-duration: $dur-loop;
    animation-timing-function: steps(1, end);
    animation-iteration-count: infinite;
  }
  // 보임은 컷(steps), 자리 잡기는 이징 — 한 요소에 두 애니메이션을 낱개 속성으로 나란히 건다
  .oh__panel--1 {
    animation-name: oh-a-on, oh-a-rise;
    animation-duration: $dur-loop, $dur-loop;
    animation-timing-function: steps(1, end), $easing;
    animation-iteration-count: infinite, infinite;
  }
  .oh__panel--2 {
    animation-name: oh-b-on, oh-b-rise;
    animation-duration: $dur-loop, $dur-loop;
    animation-timing-function: steps(1, end), $easing;
    animation-iteration-count: infinite, infinite;
  }
  .oh__shine {
    animation-name: oh-shine;
    animation-duration: $dur-loop;
    animation-timing-function: linear;
    animation-iteration-count: infinite;
    animation-fill-mode: backwards;
  }
  .oh__sk-bar:nth-child(2) .oh__shine { animation-delay: 120ms; }
  .oh__sk-bar:nth-child(3) .oh__shine { animation-delay: 240ms; }
}
// 손으로 조작할 때도 자리표시는 살아 움직인다
.oh.is-loading .oh__shine {
  animation-name: oh-shine;
  animation-duration: $dur-loop;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
}

@media (max-width: 340px) {
  .oh { gap: $sp-2; width: min(248px, 96vw); }
  .oh__title { font-size: 16px; }
  .oh__tab { padding: $sp-1 0; font-size: 11px; }
  .oh__body { height: 76px; padding: $sp-2; }
  .oh__lead { font-size: 15px; }
  .oh__sub { font-size: 11px; }
  .oh__sk-bar { height: 10px; }
}

@media (prefers-reduced-motion: reduce) {
  .oh.is-demo .oh__t--1,
  .oh.is-demo .oh__t--2,
  .oh.is-demo .oh__tab--1 .oh__pill,
  .oh.is-demo .oh__tab--2 .oh__pill,
  .oh.is-demo .oh__sk,
  .oh.is-demo .oh__panel--1,
  .oh.is-demo .oh__panel--2,
  .oh.is-demo .oh__shine,
  .oh.is-loading .oh__shine { animation: none !important; }
  // 움직임만 끄고 "이미 도착한 본문" 이라는 결과 상태는 남긴다
  .oh.is-demo .oh__t--1 { opacity: 1 !important; }
  .oh.is-demo .oh__tab--1 .oh__pill { opacity: .16 !important; }
  .oh.is-demo .oh__panel--1 { opacity: 1 !important; transform: translateY(0) !important; }
  .oh.is-demo .oh__sk { opacity: 0 !important; }
  .oh__panel { 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);
}

.oh {
  box-sizing: border-box;
  display: grid;
  gap: 12px;
  width: min(300px, 94vw);
  color: #fff7e6;
}

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

.oh__title {
  display: grid;
  margin: 0;
  font-size: 19px;
  font-weight: 800;
  line-height: 1.2;
}

.oh__t {
  grid-area: 1/1;
  opacity: 0;
}

.oh[data-tab="1"] .oh__t--1,
.oh[data-tab="2"] .oh__t--2,
.oh[data-tab="3"] .oh__t--3 {
  opacity: 1;
}

.oh__tabs {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 4px;
}

.oh__tab {
  position: relative;
  margin: 0;
  padding: 8px 0;
  border: 0;
  border-radius: 999px;
  background: none;
  color: inherit;
  font-family: inherit;
  font-size: 13px;
  font-weight: 700;
  line-height: 1;
  cursor: pointer;
}

.oh__tab:focus-visible {
  outline: 2px solid #fff7e6;
  outline-offset: 2px;
}

.oh__pill {
  position: absolute;
  inset: 0;
  border-radius: 999px;
  background: #fff7e6;
  opacity: 0;
}

.oh__lab {
  position: relative;
}

.oh[data-tab="1"] .oh__tab--1 .oh__pill,
.oh[data-tab="2"] .oh__tab--2 .oh__pill,
.oh[data-tab="3"] .oh__tab--3 .oh__pill {
  opacity: 0.16;
}

.oh__body {
  display: grid;
  height: 96px;
  padding: 12px;
  border: 1px solid rgba(255, 247, 230, 0.18);
  border-radius: 16px;
}

.oh__panel,
.oh__sk {
  grid-area: 1/1;
}

.oh__panel {
  display: grid;
  align-content: start;
  gap: 8px;
  opacity: 0;
  transform: translateY(10px);
  transition: transform 300ms cubic-bezier(0.2, 0.8, 0.2, 1), opacity 140ms linear;
}

.oh[data-tab="1"] .oh__panel--1,
.oh[data-tab="2"] .oh__panel--2,
.oh[data-tab="3"] .oh__panel--3 {
  opacity: 1;
  transform: translateY(0);
}

.oh.is-loading .oh__panel {
  opacity: 0;
  transform: translateY(10px);
}

.oh__lead {
  margin: 0;
  font-size: 17px;
  font-weight: 800;
  line-height: 1.3;
}

.oh__sub {
  margin: 0;
  color: rgba(255, 247, 230, 0.66);
  font-size: 12px;
  font-weight: 700;
  line-height: 1.4;
}

.oh__sk {
  display: grid;
  align-content: start;
  gap: 8px;
  opacity: 0;
}

.oh.is-loading .oh__sk {
  opacity: 1;
}

.oh__sk-bar {
  position: relative;
  overflow: hidden;
  height: 12px;
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.14);
}

.oh__sk-bar:nth-child(1) {
  width: 64%;
}

.oh__sk-bar:nth-child(2) {
  width: 92%;
}

.oh__sk-bar:nth-child(3) {
  width: 48%;
}

.oh__shine {
  position: absolute;
  inset: 0;
  background: linear-gradient(90deg, rgba(255, 247, 230, 0), rgba(255, 247, 230, 0.4), rgba(255, 247, 230, 0));
  transform: translateX(-100%);
}

@keyframes oh-a-on {
  0%, 9% {
    opacity: 1;
  }
  10%, 85% {
    opacity: 0;
  }
  86%, 100% {
    opacity: 1;
  }
}
@keyframes oh-b-now {
  0%, 9% {
    opacity: 0;
  }
  10%, 85% {
    opacity: 1;
  }
  86%, 100% {
    opacity: 0;
  }
}
@keyframes oh-b-on {
  0%, 39% {
    opacity: 0;
  }
  40%, 85% {
    opacity: 1;
  }
  86%, 100% {
    opacity: 0;
  }
}
@keyframes oh-pill-a {
  0%, 9% {
    opacity: 0.16;
  }
  10%, 85% {
    opacity: 0;
  }
  86%, 100% {
    opacity: 0.16;
  }
}
@keyframes oh-pill-b {
  0%, 9% {
    opacity: 0;
  }
  10%, 85% {
    opacity: 0.16;
  }
  86%, 100% {
    opacity: 0;
  }
}
@keyframes oh-sk-on {
  0%, 9% {
    opacity: 0;
  }
  10%, 39% {
    opacity: 1;
  }
  40%, 100% {
    opacity: 0;
  }
}
@keyframes oh-a-rise {
  0%, 85% {
    transform: translateY(0);
  }
  86% {
    transform: translateY(10px);
  }
  97%, 100% {
    transform: translateY(0);
  }
}
@keyframes oh-b-rise {
  0%, 40% {
    transform: translateY(10px);
  }
  64%, 85% {
    transform: translateY(0);
  }
  86%, 100% {
    transform: translateY(10px);
  }
}
@keyframes oh-shine {
  0% {
    transform: translateX(-100%);
  }
  46% {
    transform: translateX(220%);
  }
  47%, 100% {
    transform: translateX(-100%);
  }
}
.oh.is-demo .oh__t--1 {
  animation-name: oh-a-on;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.oh.is-demo .oh__t--2 {
  animation-name: oh-b-now;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.oh.is-demo .oh__tab--1 .oh__pill {
  animation-name: oh-pill-a;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.oh.is-demo .oh__tab--2 .oh__pill {
  animation-name: oh-pill-b;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.oh.is-demo .oh__sk {
  animation-name: oh-sk-on;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.oh.is-demo .oh__panel--1 {
  animation-name: oh-a-on, oh-a-rise;
  animation-duration: 2s, 2s;
  animation-timing-function: steps(1, end), cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite, infinite;
}
.oh.is-demo .oh__panel--2 {
  animation-name: oh-b-on, oh-b-rise;
  animation-duration: 2s, 2s;
  animation-timing-function: steps(1, end), cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite, infinite;
}
.oh.is-demo .oh__shine {
  animation-name: oh-shine;
  animation-duration: 2s;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
  animation-fill-mode: backwards;
}
.oh.is-demo .oh__sk-bar:nth-child(2) .oh__shine {
  animation-delay: 120ms;
}
.oh.is-demo .oh__sk-bar:nth-child(3) .oh__shine {
  animation-delay: 240ms;
}

.oh.is-loading .oh__shine {
  animation-name: oh-shine;
  animation-duration: 2s;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
}

@media (max-width: 340px) {
  .oh {
    gap: 8px;
    width: min(248px, 96vw);
  }
  .oh__title {
    font-size: 16px;
  }
  .oh__tab {
    padding: 4px 0;
    font-size: 11px;
  }
  .oh__body {
    height: 76px;
    padding: 8px;
  }
  .oh__lead {
    font-size: 15px;
  }
  .oh__sub {
    font-size: 11px;
  }
  .oh__sk-bar {
    height: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .oh.is-demo .oh__t--1,
  .oh.is-demo .oh__t--2,
  .oh.is-demo .oh__tab--1 .oh__pill,
  .oh.is-demo .oh__tab--2 .oh__pill,
  .oh.is-demo .oh__sk,
  .oh.is-demo .oh__panel--1,
  .oh.is-demo .oh__panel--2,
  .oh.is-demo .oh__shine,
  .oh.is-loading .oh__shine {
    animation: none !important;
  }
  .oh.is-demo .oh__t--1 {
    opacity: 1 !important;
  }
  .oh.is-demo .oh__tab--1 .oh__pill {
    opacity: 0.16 !important;
  }
  .oh.is-demo .oh__panel--1 {
    opacity: 1 !important;
    transform: translateY(0) !important;
  }
  .oh.is-demo .oh__sk {
    opacity: 0 !important;
  }
  .oh__panel {
    transition: none !important;
  }
}

// 낙관적 이동: 제목과 탭 표시는 누른 즉시 바뀌고, 본문만 aria-busy 를 켠 채 뒤따라온다.
var oh = document.getElementById('oh');
var ohBody = document.getElementById('oh-body');
var ohTabs = oh.querySelectorAll('.oh__tab');
var ohTimer = 0;
function ohGo(i) {
  oh.classList.remove('is-demo');
  oh.setAttribute('data-tab', String(i));
  oh.classList.add('is-loading');
  ohBody.setAttribute('aria-busy', 'true');
  ohBody.setAttribute('aria-labelledby', 'oh-tab-' + i);
  for (var k = 0; k < ohTabs.length; k++) {
    ohTabs[k].setAttribute('aria-selected', k === i - 1 ? 'true' : 'false');
  }
  clearTimeout(ohTimer);
  ohTimer = setTimeout(function () {
    oh.classList.remove('is-loading');
    ohBody.setAttribute('aria-busy', 'false');
  }, 600);
}
for (var i = 0; i < ohTabs.length; i++) {
  (function (n) {
    ohTabs[n].addEventListener('click', function () { ohGo(n + 1); });
  })(i);
}
oh.addEventListener('keydown', function (e) {
  var cur = Number(oh.getAttribute('data-tab'));
  var next = e.key === 'ArrowRight' ? cur % 3 : (e.key === 'ArrowLeft' ? (cur + 1) % 3 : -1);
  if (next < 0) return;
  e.preventDefault();
  ohGo(next + 1);
  ohTabs[next].focus({ preventScroll: true });
});

import { useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./OptimisticHeader.module.scss";

export type OptimisticTab = {
  /** 탭 줄에 보이는 짧은 이름 */
  label: string;
  /** 헤더 제목 — 탭을 누른 그 순간 바로 이 제목으로 바뀐다 */
  title: string;
  lead: string;
  sub: string;
};

export type OptimisticHeaderProps = {
  tabs?: OptimisticTab[];
  groupLabel?: string;
  tablistLabel?: string;
  /** 본문이 도착하기까지 걸리는 시간(ms) */
  delay?: number;
  /** 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 };
// ambient 가 얇아 KeyboardEvent 를 못 쓴다 — 필요한 꼴만 로컬 타입으로 선언
type KeyEvt = { key: string; preventDefault(): void };
type Box<T> = { current: T };

const DEFAULT_TABS: OptimisticTab[] = [
  { label: "Summary", title: "This week", lead: "1,284 visits", sub: "12% more than last week" },
  { label: "Activity", title: "Recent activity", lead: "7 new comments", sub: "2 posts still unanswered" },
  { label: "Alerts", title: "Alert settings", lead: "Alerts on", sub: "Weekly digest mail is on" },
];

let seq = 0;

export default function OptimisticHeader({
  tabs = DEFAULT_TABS,
  groupLabel = "Optimistic screen move",
  tablistLabel = "Pick a screen",
  delay = 600,
  demo = true,
  duration = 300,
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#fff7e6",
}: OptimisticHeaderProps) {
  const [tab, setTab] = useState(0);
  const [loading, setLoading] = useState(false);
  const [live, setLive] = useState(demo);
  const [uid] = useState<string>(() => "optimistic-header-" + String(++seq));
  const timer = useRef<number>(0) as Box<number>;
  const tabsRef = useRef<any>(null);

  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  function go(i: number) {
    setLive(false);
    setTab(i);            // 제목과 탭 표시는 누른 즉시 바뀐다 — 낙관적
    setLoading(true);     // 본문만 아직 오지 않았다
    clearTimeout(timer.current);
    timer.current = setTimeout(() => setLoading(false), delay);
  }

  function onKeyDown(e: KeyEvt) {
    const n = tabs.length;
    const next = e.key === "ArrowRight" ? (tab + 1) % n : e.key === "ArrowLeft" ? (tab + n - 1) % n : -1;
    if (next < 0) return;
    e.preventDefault();
    go(next);
    const row = tabsRef.current;
    const btn = row && row.children ? row.children[next] : null;
    if (btn) btn.focus({ preventScroll: true });
  }

  const cls = [styles.oh, live ? styles.isDemo : "", loading ? styles.isLoading : ""].filter(Boolean).join(" ");
  const bodyId = uid + "-body";

  return (
    <section className={cls} style={vars} data-tab={String(tab + 1)} aria-label={groupLabel} onKeyDown={onKeyDown}>
      <h2 className={styles.title}>
        {tabs.map((t) => (
          <span className={styles.t} key={t.title}>{t.title}</span>
        ))}
      </h2>
      <div className={styles.tabs} role="tablist" aria-label={tablistLabel} ref={tabsRef}>
        {tabs.map((t, i) => (
          <button
            className={styles.tab}
            type="button"
            role="tab"
            key={t.label}
            id={uid + "-tab-" + String(i + 1)}
            aria-controls={bodyId}
            aria-selected={i === tab}
            onClick={() => go(i)}
          >
            <span className={styles.pill} aria-hidden="true" />
            <span className={styles.lab}>{t.label}</span>
          </button>
        ))}
      </div>
      <div
        className={styles.body}
        id={bodyId}
        role="tabpanel"
        aria-labelledby={uid + "-tab-" + String(tab + 1)}
        aria-busy={loading}
      >
        {tabs.map((t) => (
          <div className={styles.panel} key={t.lead}>
            <p className={styles.lead}>{t.lead}</p>
            <p className={styles.sub}>{t.sub}</p>
          </div>
        ))}
        <div className={styles.sk} aria-hidden="true">
          <span className={styles.skBar}><span className={styles.shine} /></span>
          <span className={styles.skBar}><span className={styles.shine} /></span>
          <span className={styles.skBar}><span className={styles.shine} /></span>
        </div>
      </div>
    </section>
  );
}