08Pill that slides to the chosen slot

<main class="stage">
  <div class="sg is-demo">
    <div class="sg__bar" role="radiogroup" aria-label="Pick a billing cycle" data-at="0">
      <span class="sg__pill" aria-hidden="true"></span>
      <button class="sg__seg" type="button" role="radio" aria-checked="true" data-at="0">Monthly</button>
      <button class="sg__seg" type="button" role="radio" aria-checked="false" data-at="1">Yearly</button>
      <button class="sg__seg" type="button" role="radio" aria-checked="false" data-at="2">Lifetime</button>
    </div>
    <p class="sg__note">Pin the slot width or the pill drifts</p>
  </div>
</main>

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

$duration: $dur-base;
$easing: $ease-spring;
$color: $stage-yellow;

@include stage($stage-paper);

$segment-width: 80px;   // 세 칸을 같은 폭으로 못 박는다. 알약이 이 값의 배수만큼만 움직이므로 어긋날 자리가 없다

.sg {
  box-sizing: border-box;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.sg__bar {
  position: relative;
  box-sizing: border-box;
  display: flex;
  padding: $sp-1;
  border-radius: $r-pill;
  background-color: $subject-ink;
}
.sg__pill {
  position: absolute;
  left: 4px;
  top: 4px;
  bottom: 4px;
  width: $segment-width;
  border-radius: $r-pill;
  background-color: $color;
  transform: translateX(0);
}
.sg__seg {
  position: relative;
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  flex: 0 0 auto;          // 균등분배(flex: 1 1 0)와 섞으면 알약의 고정 이동폭과 실제 칸 폭이 어긋난다
  width: $segment-width;
  height: 36px;
  border: 0;
  border-radius: $r-pill;
  background-color: transparent;
  color: $subject-cream;
  font-family: inherit;
  font-size: 13px;
  line-height: 20px;
  font-weight: 700;
  white-space: nowrap;
  cursor: pointer;
}
.sg__seg:focus-visible { outline: 3px solid $subject-blue; outline-offset: 2px; }
.sg__seg[aria-checked="true"] { color: $subject-ink; }

// 사람이 실제로 고른 경로
.sg:not(.is-demo) .sg__pill { transition: transform $duration $easing; }
.sg:not(.is-demo) .sg__bar[data-at="1"] .sg__pill { transform: translateX($segment-width); }
.sg:not(.is-demo) .sg__bar[data-at="2"] .sg__pill { transform: translateX($segment-width * 2); }

.sg__note {
  margin: 0;
  max-width: 288px;
  text-align: center;
  font-size: 12px;
  line-height: 18px;
  font-weight: 500;
  color: $subject-ink;
}

// ── 자동 루프: 알약은 transform 으로 미끄러지고, 글자 색만 한 프레임에 끊는다 ──
.sg.is-demo .sg__pill {
  animation-name: sg-slide;
  animation-duration: $dur-loop;
  animation-timing-function: $easing;
  animation-iteration-count: infinite;
}
@keyframes sg-slide {
  0%   { transform: translateX(0); }
  18%  { transform: translateX(0); }
  34%  { transform: translateX($segment-width); }
  50%  { transform: translateX($segment-width); }
  64%  { transform: translateX($segment-width * 2); }
  76%  { transform: translateX($segment-width * 2); }
  90%  { transform: translateX(0); }
  100% { transform: translateX(0); }
}

.sg.is-demo .sg__seg {
  animation-duration: $dur-loop;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.sg.is-demo .sg__seg[data-at="0"] { animation-name: sg-a; }
.sg.is-demo .sg__seg[data-at="1"] { animation-name: sg-b; }
.sg.is-demo .sg__seg[data-at="2"] { animation-name: sg-c; }
@keyframes sg-a {
  0%   { color: $subject-ink; }
  26%  { color: $subject-cream; }
  84%  { color: $subject-ink; }
  100% { color: $subject-ink; }
}
@keyframes sg-b {
  0%   { color: $subject-cream; }
  26%  { color: $subject-ink; }
  58%  { color: $subject-cream; }
  100% { color: $subject-cream; }
}
@keyframes sg-c {
  0%   { color: $subject-cream; }
  58%  { color: $subject-ink; }
  84%  { color: $subject-cream; }
  100% { color: $subject-cream; }
}

@media (max-width: 340px) {
  .sg { gap: $sp-2; }
  .sg__note { font-size: 11px; line-height: 16px; max-width: 200px; }
}

@media (prefers-reduced-motion: reduce) {
  .sg.is-demo .sg__pill,
  .sg.is-demo .sg__seg { animation: none !important; }
  .sg:not(.is-demo) .sg__pill { transition: none !important; }
  .sg.is-demo .sg__seg[data-at="0"] { color: $subject-ink; }
}

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

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

.sg {
  box-sizing: border-box;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.sg__bar {
  position: relative;
  box-sizing: border-box;
  display: flex;
  padding: 4px;
  border-radius: 999px;
  background-color: #17141a;
}

.sg__pill {
  position: absolute;
  left: 4px;
  top: 4px;
  bottom: 4px;
  width: 80px;
  border-radius: 999px;
  background-color: #ffd23f;
  transform: translateX(0);
}

.sg__seg {
  position: relative;
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  flex: 0 0 auto;
  width: 80px;
  height: 36px;
  border: 0;
  border-radius: 999px;
  background-color: transparent;
  color: #fff7e6;
  font-family: inherit;
  font-size: 13px;
  line-height: 20px;
  font-weight: 700;
  white-space: nowrap;
  cursor: pointer;
}

.sg__seg:focus-visible {
  outline: 3px solid #2f6df6;
  outline-offset: 2px;
}

.sg__seg[aria-checked=true] {
  color: #17141a;
}

.sg:not(.is-demo) .sg__pill {
  transition: transform 300ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

.sg:not(.is-demo) .sg__bar[data-at="1"] .sg__pill {
  transform: translateX(80px);
}

.sg:not(.is-demo) .sg__bar[data-at="2"] .sg__pill {
  transform: translateX(160px);
}

.sg__note {
  margin: 0;
  max-width: 288px;
  text-align: center;
  font-size: 12px;
  line-height: 18px;
  font-weight: 500;
  color: #17141a;
}

.sg.is-demo .sg__pill {
  animation-name: sg-slide;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite;
}

@keyframes sg-slide {
  0% {
    transform: translateX(0);
  }
  18% {
    transform: translateX(0);
  }
  34% {
    transform: translateX(80px);
  }
  50% {
    transform: translateX(80px);
  }
  64% {
    transform: translateX(160px);
  }
  76% {
    transform: translateX(160px);
  }
  90% {
    transform: translateX(0);
  }
  100% {
    transform: translateX(0);
  }
}
.sg.is-demo .sg__seg {
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}

.sg.is-demo .sg__seg[data-at="0"] {
  animation-name: sg-a;
}

.sg.is-demo .sg__seg[data-at="1"] {
  animation-name: sg-b;
}

.sg.is-demo .sg__seg[data-at="2"] {
  animation-name: sg-c;
}

@keyframes sg-a {
  0% {
    color: #17141a;
  }
  26% {
    color: #fff7e6;
  }
  84% {
    color: #17141a;
  }
  100% {
    color: #17141a;
  }
}
@keyframes sg-b {
  0% {
    color: #fff7e6;
  }
  26% {
    color: #17141a;
  }
  58% {
    color: #fff7e6;
  }
  100% {
    color: #fff7e6;
  }
}
@keyframes sg-c {
  0% {
    color: #fff7e6;
  }
  58% {
    color: #17141a;
  }
  84% {
    color: #fff7e6;
  }
  100% {
    color: #fff7e6;
  }
}
@media (max-width: 340px) {
  .sg {
    gap: 8px;
  }
  .sg__note {
    font-size: 11px;
    line-height: 16px;
    max-width: 200px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sg.is-demo .sg__pill,
  .sg.is-demo .sg__seg {
    animation: none !important;
  }
  .sg:not(.is-demo) .sg__pill {
    transition: none !important;
  }
  .sg.is-demo .sg__seg[data-at="0"] {
    color: #17141a;
  }
}

// 실제 동작: 고른 칸은 aria-checked 로 적고, 알약은 칸 번호 × 칸 폭만큼만 옮긴다
var root = document.querySelector('.sg');
var bar = root.querySelector('.sg__bar');
var segs = [].slice.call(root.querySelectorAll('.sg__seg'));
function select(i) {
  bar.setAttribute('data-at', String(i));
  segs.forEach(function (b, k) { b.setAttribute('aria-checked', String(k === i)); });
}
segs.forEach(function (b, i) {
  b.addEventListener('click', function () { root.classList.remove('is-demo'); select(i); });
});
bar.addEventListener('keydown', function (e) {
  var i = segs.indexOf(document.activeElement);
  if (i < 0 || (e.key !== 'ArrowRight' && e.key !== 'ArrowLeft')) return;
  e.preventDefault();
  root.classList.remove('is-demo');
  var n = (i + (e.key === 'ArrowRight' ? 1 : 2)) % 3;
  select(n);
  segs[n].focus({ preventScroll: true });   // iframe 안에서 부모 페이지가 끌려가지 않게
});

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

export type SegmentedRadioGroupProps = {
  labels?: [string, string, string];   // 알약 이동폭이 80px 고정이라 칸은 세 개다
  groupLabel?: string;
  note?: string;
  duration?: number;
  easing?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string };
// DOM 전역 타입을 react 에서 가져오지 않는다 — 쓰는 속성만 여기에 적는다
type KeyEvt = { key: string; preventDefault(): void };
type Focusable = { focus(opt: { preventScroll: boolean }): void };

export default function SegmentedRadioGroup({
  labels = ["Monthly", "Yearly", "Lifetime"],
  groupLabel = "Choose a billing cycle",
  note = "Pin the segment width so the pill never drifts",
  duration = 300,
  easing = "cubic-bezier(.2,.8,.2,1)",
}: SegmentedRadioGroupProps) {
  const [demo, setDemo] = useState(true); // 처음엔 자동 루프, 사람이 건드리면 끈다
  const [at, setAt] = useState(0);
  const segs = useRef<(Focusable | null)[]>([]);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing };

  const pick = useCallback((i: number) => {
    setDemo(false);
    setAt(i);
  }, []);

  // 좌우 화살표로 이동 — 활성 요소를 뒤지지 않고 칸 번호를 그대로 쓴다
  const onKeyDown = useCallback((e: KeyEvt, i: number) => {
    if (e.key !== "ArrowRight" && e.key !== "ArrowLeft") return;
    e.preventDefault();
    setDemo(false);
    const n = (i + (e.key === "ArrowRight" ? 1 : labels.length - 1)) % labels.length;
    setAt(n);
    // preventScroll — iframe 안에서 부모 페이지가 끌려가지 않게
    const segEls = segs.current;
    if (segEls) segEls[n]?.focus({ preventScroll: true });
  }, [labels.length]);

  return (
    <div className={`${styles.sg} ${demo ? styles.isDemo : ""}`} style={vars}>
      <div className={styles.bar} role="radiogroup" aria-label={groupLabel} data-at={String(at)}>
        <span className={styles.pill} aria-hidden="true" />
        {labels.map((label, i) => (
          <button
            key={label}
            ref={(el: Focusable | null) => { const segEls = segs.current; if (segEls) segEls[i] = el; }}
            className={styles.seg}
            type="button"
            role="radio"
            aria-checked={at === i}
            data-at={String(i)}
            onClick={() => pick(i)}
            onKeyDown={(e: KeyEvt) => onKeyDown(e, i)}
          >
            {label}
          </button>
        ))}
      </div>
      <p className={styles.note}>{note}</p>
    </div>
  );
}