04サイド縦バー

<main class="stage">
  <div class="rail" role="progressbar" aria-label="読んでいる位置" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0">
    <div class="rail__fill is-demo"></div>
  </div>
  <div class="page">
    <div class="page__block"></div>
    <div class="page__block"></div>
    <div class="page__block short"></div>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$thickness: 4px;                                    // 레일 두께
$color: $subject-blue;                              // 채워진 구간 색
$track: rgba(23, 20, 26, .1);                       // 안 읽은 구간

@include stage($stage-paper);

.rail {
  position: fixed;
  top: 0; right: 0;
  width: $thickness;
  height: 100%;
  background: $track;
}

.rail__fill {
  width: 100%;
  height: 100%;
  background: $color;
  transform: scaleY(0);
  transform-origin: top; // 규칙8: 세로레일 top 채움

  // 실제 사용: 문서 안에서 지금까지 읽은 위치를 세로로 보여준다
  @supports (animation-timeline: scroll()) {
    animation: fill-y linear both;
    animation-timeline: scroll(root);
  }
}
@keyframes fill-y { to { transform: scaleY(1); } }

.rail__fill.is-demo { animation: fill-y-loop $dur-loop linear infinite; }
@keyframes fill-y-loop {
  0%   { transform: scaleY(0); }
  92%  { transform: scaleY(1); }
  100% { transform: scaleY(1); }
}

.page { padding: $sp-10 $sp-8 0 $sp-6; }
.page__block {
  height: $sp-6;
  margin: 0 0 $sp-4;
  border-radius: $r-sm;
  background: rgba(23, 20, 26, .08);
  &.short { width: 55%; }
}

@media (prefers-reduced-motion: reduce) {
  .rail__fill, .rail__fill.is-demo { animation: none !important; transform: scaleY(1); }
}

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);
}

.rail {
  position: fixed;
  top: 0;
  right: 0;
  width: 4px;
  height: 100%;
  background: rgba(23, 20, 26, 0.1);
}

.rail__fill {
  width: 100%;
  height: 100%;
  background: #2f6df6;
  transform: scaleY(0);
  transform-origin: top;
}
@supports (animation-timeline: scroll()) {
  .rail__fill {
    animation: fill-y linear both;
    animation-timeline: scroll(root);
  }
}

@keyframes fill-y {
  to {
    transform: scaleY(1);
  }
}
.rail__fill.is-demo {
  animation: fill-y-loop 2s linear infinite;
}

@keyframes fill-y-loop {
  0% {
    transform: scaleY(0);
  }
  92% {
    transform: scaleY(1);
  }
  100% {
    transform: scaleY(1);
  }
}
.page {
  padding: 40px 32px 0 24px;
}

.page__block {
  height: 24px;
  margin: 0 0 16px;
  border-radius: 8px;
  background: rgba(23, 20, 26, 0.08);
}
.page__block.short {
  width: 55%;
}

@media (prefers-reduced-motion: reduce) {
  .rail__fill, .rail__fill.is-demo {
    animation: none !important;
    transform: scaleY(1);
  }
}

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

export type SideRailProps = {
  /** 레일 두께 (기본 4px) */
  thickness?: number;
  /** 채워진 구간 색 */
  color?: string;
};

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

// 실제 쓰임: TopBar 와 같은 계산을 세로(scaleY)로 그린다.
export default function SideRail({ thickness = 4, color = "#2f6df6" }: SideRailProps) {
  const [scale, setScale] = useState(0);

  useEffect(() => {
    const onScroll = () => {
      const max = document.documentElement.scrollHeight - window.innerHeight;
      setScale(max > 0 ? Math.min(1, window.scrollY / max) : 0);
    };
    onScroll();
    window.addEventListener("scroll", onScroll, { passive: true });
    return () => window.removeEventListener("scroll", onScroll);
  }, []);

  const vars: Vars = { "--thickness": `${thickness}px`, "--color": color };

  return (
    <div className={styles.rail} style={vars} role="progressbar" aria-label="Reading position"
         aria-valuemin={0} aria-valuemax={100} aria-valuenow={Math.round(scale * 100)}>
      <div className={styles.fill} style={{ transform: `scaleY(${scale})` }} />
    </div>
  );
}