07Starts only when scrolled into view

<main class="stage">
  <div class="viewport is-demo">
    <div class="track">
      <div class="card">
        <span class="card__num" aria-hidden="true"></span>
        <span class="card__sr">1,280</span>
        <span class="card__label">Total visitors</span>
      </div>
    </div>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;
$easing: $ease-out;
$color: $subject-cream;

@include stage($stage-orange);

@property --num {
  syntax: '<integer>';
  initial-value: 0;
  inherits: false;
}

.viewport {
  position: relative;
  width: min(100%, 220px);
  height: clamp(140px, 38vw, 170px);
  overflow: hidden;                                  // 뷰포트 밖 카드는 안 보인다 — 실제 화면 가장자리 역할
  border-radius: $r-card;
  background: rgba(255, 255, 255, .08);
}

.track {
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
}

.card {
  text-align: center;

  &__num {
    --num: 0;
    counter-reset: num var(--num);
    display: block;
    font-weight: 800; font-size: clamp(24px, 7vw, 34px); line-height: 1; font-family: inherit;
    font-variant-numeric: tabular-nums;
    color: $color;

    &::before { content: counter(num); }
  }

  &__sr {
    position: absolute;
    width: 1px; height: 1px;
    overflow: hidden;
    clip-path: inset(50%);
    white-space: nowrap;
  }

  &__label {
    display: block;
    margin-top: 6px;
    font-weight: 600; font-size: clamp(11px, 3.2vw, 13px); line-height: 1; font-family: inherit;
    color: $subject-ink;                             // 주황 면 위 크림 글자는 2.28:1 — 잉크로 5.90:1
  }

  // 실제 페이지: JS 가 붙인 .is-counting 이 있어야 카운트가 돈다 (뷰포트 밖이면 0에 멈춰 있다)
  &__num { animation-play-state: paused; }
  &.is-counting &__num { animation-play-state: running; }
}

// ── 데모(.is-demo): 카드가 스크롤하듯 아래→가운데→위로 지나가고, 가운데 있는 동안만 카운트가 돈다 ──
.viewport.is-demo .track {
  animation: scroll-pass $duration $easing infinite;
}
.viewport.is-demo .card__num {
  animation: count-when-visible $duration $easing infinite;
}

@keyframes scroll-pass {
  0%       { transform: translateY(64px); }
  20%, 75% { transform: translateY(0); }
  100%     { transform: translateY(-64px); }
}
@keyframes count-when-visible {
  0%, 20% { --num: 0; }
  60%     { --num: 1280; }
  100%    { --num: 1280; }
}

@media (prefers-reduced-motion: reduce) {
  .viewport.is-demo .track { animation: none; transform: translateY(0); }
  .viewport.is-demo .card__num { animation: none; --num: 1280; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

@property --num {
  syntax: "<integer>";
  initial-value: 0;
  inherits: false;
}
.viewport {
  position: relative;
  width: min(100%, 220px);
  height: clamp(140px, 38vw, 170px);
  overflow: hidden;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.08);
}

.track {
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
}

.card {
  text-align: center;
}
.card__num {
  --num: 0;
  counter-reset: num var(--num);
  display: block;
  font-weight: 800;
  font-size: clamp(24px, 7vw, 34px);
  line-height: 1;
  font-family: inherit;
  font-variant-numeric: tabular-nums;
  color: #fff7e6;
}
.card__num::before {
  content: counter(num);
}
.card__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}
.card__label {
  display: block;
  margin-top: 6px;
  font-weight: 600;
  font-size: clamp(11px, 3.2vw, 13px);
  line-height: 1;
  font-family: inherit;
  color: #17141a;
}
.card__num {
  animation-play-state: paused;
}
.card.is-counting .card__num {
  animation-play-state: running;
}

.viewport.is-demo .track {
  animation: scroll-pass 2s ease-out infinite;
}

.viewport.is-demo .card__num {
  animation: count-when-visible 2s ease-out infinite;
}

@keyframes scroll-pass {
  0% {
    transform: translateY(64px);
  }
  20%, 75% {
    transform: translateY(0);
  }
  100% {
    transform: translateY(-64px);
  }
}
@keyframes count-when-visible {
  0%, 20% {
    --num: 0;
  }
  60% {
    --num: 1280;
  }
  100% {
    --num: 1280;
  }
}
@media (prefers-reduced-motion: reduce) {
  .viewport.is-demo .track {
    animation: none;
    transform: translateY(0);
  }
  .viewport.is-demo .card__num {
    animation: none;
    --num: 1280;
  }
}

// 실제 페이지: 카드가 뷰포트에 60% 이상 들어와야 카운트를 시작한다 (스크롤해서 보이기 전엔 0)
var card = document.querySelector('.card');
new IntersectionObserver(function (es) {
  card.classList.toggle('is-counting', es[0].isIntersecting);
}, { threshold: .6 }).observe(card);

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

export type CounterOnViewStartProps = {
  value: number;
  label: string;
  duration?: number;
  easing?: string;
  color?: string;
};

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

export default function CounterOnViewStart({
  value, label, duration = 1200, easing = "ease-out", color = "#fff7e6",
}: CounterOnViewStartProps) {
  const ref = useRef<HTMLDivElement>(null);
  const [counting, setCounting] = useState(false);

  useEffect(() => {
    const el = ref.current;
    if (!el) return;
    const io = new IntersectionObserver(
      (entries) => setCounting(entries[0].isIntersecting),
      { threshold: .6 },
    );
    io.observe(el);
    return () => io.disconnect();
  }, []);

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

  return (
    <div ref={ref} className={`${styles.card} ${counting ? styles.isCounting : ""}`} style={vars}>
      <span className={styles.num} aria-hidden="true" />
      <span className={styles.sr}>{value.toLocaleString()}</span>
      <span className={styles.label}>{label}</span>
    </div>
  );
}