05Chữ được tô đầy dần

<main class="stage">
  <h2 class="headline is-demo">Tiêu đề được tô đầy theo mức đã đọc</h2>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$color: $subject-blue;                              // 읽은 구간 색
$dim: rgba(255, 255, 255, .32);                     // 안 읽은 구간 색
$duration: $dur-loop;

@include stage($stage-ink);

// 퍼센트라야 그라데이션 경계가 매끄럽게 보간된다
@property --p {
  syntax: '<percentage>';
  initial-value: 0%;
  inherits: false;
}

.headline {
  --p: 0%;
  max-width: 80%;
  margin: 0;
  text-align: center;
  font-weight: 800;
  font-size: clamp(20px, 6vw, 28px);
  line-height: 1.35;
  font-family: inherit;
  background-image: linear-gradient(90deg, $color var(--p), $dim var(--p));
  background-clip: text;
  -webkit-background-clip: text;
  color: transparent;

  // 실제 사용: 헤드라인 색이 스크롤 진행률만큼 왼쪽부터 채워진다
  @supports (animation-timeline: scroll()) {
    animation: fill-text linear both;
    animation-timeline: scroll(root);
  }
}
@keyframes fill-text { from { --p: 0%; } to { --p: 100%; } }

.headline.is-demo { animation: fill-text-loop $duration linear infinite; }
@keyframes fill-text-loop {
  0%   { --p: 0%; }
  92%  { --p: 100%; }
  100% { --p: 100%; }
}

@media (prefers-reduced-motion: reduce) {
  .headline, .headline.is-demo { animation: none !important; --p: 100%; }
}

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);
}

@property --p {
  syntax: "<percentage>";
  initial-value: 0%;
  inherits: false;
}
.headline {
  --p: 0%;
  max-width: 80%;
  margin: 0;
  text-align: center;
  font-weight: 800;
  font-size: clamp(20px, 6vw, 28px);
  line-height: 1.35;
  font-family: inherit;
  background-image: linear-gradient(90deg, #2f6df6 var(--p), rgba(255, 255, 255, 0.32) var(--p));
  background-clip: text;
  -webkit-background-clip: text;
  color: transparent;
}
@supports (animation-timeline: scroll()) {
  .headline {
    animation: fill-text linear both;
    animation-timeline: scroll(root);
  }
}

@keyframes fill-text {
  from {
    --p: 0%;
  }
  to {
    --p: 100%;
  }
}
.headline.is-demo {
  animation: fill-text-loop 2s linear infinite;
}

@keyframes fill-text-loop {
  0% {
    --p: 0%;
  }
  92% {
    --p: 100%;
  }
  100% {
    --p: 100%;
  }
}
@media (prefers-reduced-motion: reduce) {
  .headline, .headline.is-demo {
    animation: none !important;
    --p: 100%;
  }
}

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

export type TextFillProps = {
  children: ReactNode;
  /** 읽은 구간 색 */
  color?: string;
  /** 안 읽은 구간 색 */
  dim?: string;
};

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

// 실제 쓰임: 헤드라인 색이 스크롤 진행률만큼 왼쪽부터 채워진다.
export default function TextFill({ children, color = "#2f6df6", dim = "rgba(255,255,255,.32)" }: TextFillProps) {
  const [percent, setPercent] = useState(0);

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

  const vars: Vars = { "--p": `${percent}%`, "--color": color, "--dim": dim };

  return <h2 className={styles.headline} style={vars}>{children}</h2>;
}