01Seamless loop formula

<main class="stage">
  <div class="marquee">
    <div class="marquee__track">
      <span class="marquee__chip">A</span>
      <span class="marquee__chip">B</span>
      <span class="marquee__chip">C</span>
      <span class="marquee__chip">D</span>
      <span class="marquee__chip">E</span>
      <span class="marquee__chip">F</span>
      <span class="marquee__chip" aria-hidden="true">A</span>
      <span class="marquee__chip" aria-hidden="true">B</span>
      <span class="marquee__chip" aria-hidden="true">C</span>
      <span class="marquee__chip" aria-hidden="true">D</span>
      <span class="marquee__chip" aria-hidden="true">E</span>
      <span class="marquee__chip" aria-hidden="true">F</span>
    </div>
  </div>
</main>

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

$duration: $dur-loop;                              // 실제 서비스는 12~20s 가 보통. 데모는 gif 루프에 맞춰 2s 로 배속
$easing: linear;                                    // 등속 — 완급을 준 버전은 07
$color: $subject-cream;

@include stage($stage-ink);

.marquee {
  width: min(320px, 84vw);
  overflow: hidden;
}
.marquee__track {
  display: flex;
  width: max-content;
  animation: marquee-scroll $duration $easing infinite;
}
.marquee__chip {
  margin-right: 14px;   // 트랙의 gap 대신 — 세트 폭이 정확히 절반이 되어 -50% 이음새가 0px 이 된다
  display: grid;
  place-items: center;
  flex-shrink: 0;
  width: 40px;
  height: 40px;
  border-radius: $r-control;
  background: $color;
  color: $stage-ink;
  font-weight: 700; font-size: 16px; line-height: 1; font-family: inherit;
}

// 6개를 그대로 한 번 더 복제해 트랙 하나에 12개를 나란히 두었다.
// 간격은 트랙의 gap 이 아니라 칩의 margin-right 로 준다. gap 은 12개 사이에 11번만 생겨
// -50% 가 gap 의 절반만큼 모자라고, 한 바퀴마다 그만큼 튄다 (이 편 9개 전부에서 4~20px 실측).
// margin-right 면 세트 하나의 폭 = (칩 + 간격) × 6 이라 -50% 가 픽셀 단위로 겹친다.
@keyframes marquee-scroll {
  from { transform: translateX(0); }
  to   { transform: translateX(-50%); }
}

@media (prefers-reduced-motion: reduce) {
  .marquee__track { animation: none; transform: translateX(0); }
}

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);
}

.marquee {
  width: min(320px, 84vw);
  overflow: hidden;
}

.marquee__track {
  display: flex;
  width: max-content;
  animation: marquee-scroll 2s linear infinite;
}

.marquee__chip {
  margin-right: 14px;
  display: grid;
  place-items: center;
  flex-shrink: 0;
  width: 40px;
  height: 40px;
  border-radius: 12px;
  background: #fff7e6;
  color: #17141a;
  font-weight: 700;
  font-size: 16px;
  line-height: 1;
  font-family: inherit;
}

@keyframes marquee-scroll {
  from {
    transform: translateX(0);
  }
  to {
    transform: translateX(-50%);
  }
}
@media (prefers-reduced-motion: reduce) {
  .marquee__track {
    animation: none;
    transform: translateX(0);
  }
}

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

export type SeamlessLoopMarqueeProps = {
  items?: string[];
  /** 한 바퀴가 도는 시간 (기본 12000ms — 데모 gif 는 2000ms 로 배속) */
  duration?: number;
  easing?: string;
  color?: string;
};

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

export default function SeamlessLoopMarquee({
  items = ["A", "B", "C", "D", "E", "F"],
  duration = 12000,
  easing = "linear",
  color = "#fff7e6",
}: SeamlessLoopMarqueeProps) {
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  return (
    <div className={styles.marquee} style={vars}>
      <div className={styles.track}>
        {items.map((it) => (
          <span key={it} className={styles.chip}>{it}</span>
        ))}
        {items.map((it) => (
          <span key={"dup-" + it} className={styles.chip} aria-hidden="true">{it}</span>
        ))}
      </div>
    </div>
  );
}