03Circular progress ring

<main class="stage">
  <button type="button" class="totop is-demo" role="progressbar" aria-label="Read progress" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0">
    <svg class="totop__ring" viewBox="0 0 56 56" aria-hidden="true">
      <circle class="totop__track" cx="28" cy="28" r="24"/>
      <circle class="totop__fill" cx="28" cy="28" r="24"/>
    </svg>
    <svg class="totop__arrow" viewBox="0 0 24 24" aria-hidden="true"><polyline points="5,15 12,8 19,15"/></svg>
  </button>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;
$easing: linear;
$color: $subject-ink;      // 링 색

@include stage($stage-orange);

// 정수라야 링이 매끄럽게 보간된다 (047 스크롤 진행 배지와 같은 기법)
// inherits:true — .totop 에서 애니메이션하고 자식 SVG 원(.totop__fill)이 그 값을 물려받아 쓴다
@property --p {
  syntax: '<number>';
  initial-value: 0;
  inherits: true;
}

.totop {
  --p: 0;
  position: relative;
  width: 56px; height: 56px;
  border: 0;
  border-radius: $r-pill;
  background: $stage-paper;
  display: grid;
  place-items: center;
  box-shadow: $shadow-raised;
  cursor: pointer;
}
// 데모 루프 — 실제로는 JS 가 스크롤 비율을 --p 에 직접 대입하지만, 여기선 0→100 을 자동으로 돈다
.totop.is-demo { animation: totop-fill $duration $easing infinite; }
@keyframes totop-fill { from { --p: 0; } to { --p: 100; } }

.totop__ring {
  position: absolute;
  inset: 0;
  width: 56px; height: 56px;
  transform: rotate(-90deg);
}
.totop__track, .totop__fill {
  fill: none;
  stroke-width: 3.4;
}
.totop__track { stroke: rgba($stage-ink, .14); }
.totop__fill {
  stroke: $color;
  stroke-linecap: round;
  stroke-dasharray: 150.8px;
  stroke-dashoffset: calc(150.8px * (1 - var(--p) / 100));
}

.totop__arrow {
  width: 20px; height: 20px;
  fill: none;
  stroke: $color;
  stroke-width: 2.4;
  stroke-linecap: round;
  stroke-linejoin: round;
}

@media (prefers-reduced-motion: reduce) {
  .totop.is-demo { animation: none; --p: 100; }
}

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 --p {
  syntax: "<number>";
  initial-value: 0;
  inherits: true;
}
.totop {
  --p: 0;
  position: relative;
  width: 56px;
  height: 56px;
  border: 0;
  border-radius: 999px;
  background: #ffffff;
  display: grid;
  place-items: center;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  cursor: pointer;
}

.totop.is-demo {
  animation: totop-fill 2s linear infinite;
}

@keyframes totop-fill {
  from {
    --p: 0;
  }
  to {
    --p: 100;
  }
}
.totop__ring {
  position: absolute;
  inset: 0;
  width: 56px;
  height: 56px;
  transform: rotate(-90deg);
}

.totop__track, .totop__fill {
  fill: none;
  stroke-width: 3.4;
}

.totop__track {
  stroke: rgba(23, 20, 26, 0.14);
}

.totop__fill {
  stroke: #17141a;
  stroke-linecap: round;
  stroke-dasharray: 150.8px;
  stroke-dashoffset: calc(150.8px * (1 - var(--p) / 100));
}

.totop__arrow {
  width: 20px;
  height: 20px;
  fill: none;
  stroke: #17141a;
  stroke-width: 2.4;
  stroke-linecap: round;
  stroke-linejoin: round;
}

@media (prefers-reduced-motion: reduce) {
  .totop.is-demo {
    animation: none;
    --p: 100;
  }
}

// 실제 사용: 스크롤 비율을 계산해 --p 에 직접 대입한다 (7줄). 데모 루프는 별도 keyframes 가 담당한다
var el = document.querySelector('.totop');
function sync() {
  var max = document.documentElement.scrollHeight - window.innerHeight;
  var ratio = max > 0 ? Math.min(100, (window.scrollY / max) * 100) : 0;
  el.style.setProperty('--p', ratio);
}
window.addEventListener('scroll', sync, { passive: true });

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

export type CircleProgressButtonProps = {
  color?: string;
};

type Vars = CSSProperties & { "--p"?: number };

// 실제 사용: 스크롤 비율을 계산해 --p 에 직접 대입한다. 애니메이션이 아니라 값 그대로 반영한다
export default function CircleProgressButton({ color = "#17141a" }: CircleProgressButtonProps) {
  const [p, setP] = useState(0);

  useEffect(() => {
    const sync = () => {
      const max = document.documentElement.scrollHeight - window.innerHeight;
      setP(max > 0 ? Math.min(100, (window.scrollY / max) * 100) : 0);
    };
    sync();
    window.addEventListener("scroll", sync, { passive: true });
    return () => window.removeEventListener("scroll", sync);
  }, []);

  const vars: Vars = { "--p": p };
  const circumference = 150.8;

  return (
    <button
      type="button"
      className={styles.totop}
      style={{ ...vars, color }}
      role="progressbar"
      aria-label="읽은 비율"
      aria-valuemin={0}
      aria-valuemax={100}
      aria-valuenow={Math.round(p)}
      onClick={() => window.scrollTo({ top: 0, behavior: "smooth" })}
    >
      <svg className={styles.ring} viewBox="0 0 56 56" aria-hidden="true">
        <circle className={styles.track} cx="28" cy="28" r="24" />
        <circle
          className={styles.fill}
          cx="28"
          cy="28"
          r="24"
          style={{ strokeDasharray: circumference, strokeDashoffset: circumference * (1 - p / 100) }}
        />
      </svg>
      <svg className={styles.arrow} viewBox="0 0 24 24" aria-hidden="true">
        <polyline points="5,15 12,8 19,15" />
      </svg>
    </button>
  );
}