09Trượt mượt bằng scroll-behavior

<main class="stage">
  <div class="mini-page" aria-hidden="true">
    <div class="mini-page__track is-demo">
      <span id="mp-top" class="mini-page__line mini-page__line--mark"></span>
      <span class="mini-page__line"></span>
      <span class="mini-page__line"></span>
      <span class="mini-page__line"></span>
      <span class="mini-page__line"></span>
      <span class="mini-page__line"></span>
    </div>
  </div>
  <a href="#mp-top" class="totop" aria-label="Lên đầu trang">
    <svg class="totop__arrow" viewBox="0 0 24 24" aria-hidden="true"><polyline points="5,15 12,8 19,15"/></svg>
  </a>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;
$easing: ease-in-out;      // 브라우저 scroll-behavior:smooth 의 기본 이징과 가장 가깝다
$color: $subject-ink;      // 화살표 색

@include stage($stage-ink);
.stage { position: relative; }

// 실제 사용: html { scroll-behavior: smooth; } 한 줄 + <a href="#mp-top"> 뿐이라 여기엔 JS 가 없다.
// 이 박스는 480×300 안에서 실제 문서 스크롤을 흉내 내려고 translateY 트랙만 옮긴다(050 의 scrollLeft 교훈과 같은 계열).
.mini-page {
  position: absolute;
  inset: $sp-6 $sp-12 $sp-6 $sp-6;
  overflow: hidden;
  border-radius: $r-card;
  background: rgba(#fff, .06);
}
.mini-page__track {
  display: flex;
  flex-direction: column;
  gap: $sp-4;
  padding: $sp-4;
  transform: translateY(-140px);
}
.mini-page__track.is-demo { animation: mp-scroll $duration $easing infinite; }
@keyframes mp-scroll {
  0%, 25%  { transform: translateY(-140px); }
  70%, 100% { transform: translateY(0); }
}

.mini-page__line {
  display: block;
  height: $sp-4;
  border-radius: $r-xs;
  background: rgba(#fff, .16);
}
.mini-page__line--mark { background: $stage-yellow; }

.totop {
  position: absolute;
  right: $sp-6; bottom: $sp-6;
  width: 48px; height: 48px;
  border-radius: $r-pill;
  background: $stage-yellow;
  display: grid;
  place-items: center;
  box-shadow: $shadow-raised;
  text-decoration: none;
}

.totop__arrow {
  width: 22px; height: 22px;
  fill: none;
  stroke: $color;
  stroke-width: 2.4;
  stroke-linecap: round;
  stroke-linejoin: round;
}

@media (prefers-reduced-motion: reduce) {
  .mini-page__track.is-demo { animation: none; transform: translateY(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);
}

.stage {
  position: relative;
}

.mini-page {
  position: absolute;
  inset: 24px 48px 24px 24px;
  overflow: hidden;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.06);
}

.mini-page__track {
  display: flex;
  flex-direction: column;
  gap: 16px;
  padding: 16px;
  transform: translateY(-140px);
}

.mini-page__track.is-demo {
  animation: mp-scroll 2s ease-in-out infinite;
}

@keyframes mp-scroll {
  0%, 25% {
    transform: translateY(-140px);
  }
  70%, 100% {
    transform: translateY(0);
  }
}
.mini-page__line {
  display: block;
  height: 16px;
  border-radius: 4px;
  background: rgba(255, 255, 255, 0.16);
}

.mini-page__line--mark {
  background: #ffd23f;
}

.totop {
  position: absolute;
  right: 24px;
  bottom: 24px;
  width: 48px;
  height: 48px;
  border-radius: 999px;
  background: #ffd23f;
  display: grid;
  place-items: center;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  text-decoration: none;
}

.totop__arrow {
  width: 22px;
  height: 22px;
  fill: none;
  stroke: #17141a;
  stroke-width: 2.4;
  stroke-linecap: round;
  stroke-linejoin: round;
}

@media (prefers-reduced-motion: reduce) {
  .mini-page__track.is-demo {
    animation: none;
    transform: translateY(0);
  }
}

import styles from "./SmoothAnchorButton.module.scss";

export type SmoothAnchorButtonProps = {
  /** 스크롤 최상단에 둘 앵커 id (기본 "top") */
  targetId?: string;
};

// 실제 사용: 전역 CSS 에 html { scroll-behavior: smooth; } 한 줄만 있으면 된다. JS 는 0줄이다
export default function SmoothAnchorButton({ targetId = "top" }: SmoothAnchorButtonProps) {
  return (
    <a href={`#${targetId}`} className={styles.totop} aria-label="맨 위로">
      <svg className={styles.arrow} viewBox="0 0 24 24" aria-hidden="true">
        <polyline points="5,15 12,8 19,15" />
      </svg>
    </a>
  );
}