03スクロール位置に応じて真ん中から両側へ描かれる区切り線

<main class="stage">
<div class="dv is-demo" id="dvRoot">
  <div class="dv__scroller" id="dvScroller" tabindex="0" aria-label="スクロール領域">
    <div class="dv__block">上の段落</div>
    <div class="dv__block dv__block--tall">上の段落</div>
    <div class="dv__line" id="dvLine" role="separator"></div>
    <div class="dv__block">下の段落</div>
    <div class="dv__block dv__block--tall">下の段落</div>
  </div>
  <p class="dv__meta"><span class="dv__tag">DRAW</span> <b id="dvPct">0</b>%</p>
</div>
</main>

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

// 핵심: 스크롤로 올라온 거리(DRAW_RANGE_PX 기준)를 clip-path inset 으로 바꿔 선이 가운데에서 양쪽으로 열린다.

$line-h: 10px;
$line-color: $subject-mint;

@include stage($stage-ink);

.dv {
  width: min(100%, 360px);
  display: grid;
  gap: $sp-3;
}

.dv__scroller {
  box-sizing: border-box;
  height: 128px;
  overflow-y: auto;
  scrollbar-width: none;
  display: grid;
  gap: $sp-3;
  align-content: start;
  padding: $sp-3;
  border-radius: $r-card;
  background: $tick-panel;
}
.dv__scroller::-webkit-scrollbar { display: none; }
.dv__scroller:focus-visible { outline: 3px solid $stage-paper; outline-offset: 3px; }

.dv__block {
  box-sizing: border-box;
  height: 40px;
  padding: 0 $sp-4;
  display: flex;
  align-items: center;
  border-radius: $r-control;
  background: rgba($subject-lilac, .22);
  color: $stage-paper;
  font-size: 14px;
  font-weight: 700;
  line-height: 20px;
}
.dv__block--tall { height: 72px; }

.dv__line {
  flex: none;
  height: $line-h;
  background: $line-color;
  clip-path: inset(0 50% 0 50% round 6px);
}

.dv__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: rgba($stage-paper, .78);
}
.dv__tag { font-weight: 700; letter-spacing: .08em; }
.dv__meta b { min-width: 3ch; text-align: right; font-variant-numeric: tabular-nums; color: $stage-paper; }

@media (prefers-reduced-motion: reduce) {
  .dv__line { transition: none; }
}

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);
}

.dv {
  width: min(100%, 360px);
  display: grid;
  gap: 12px;
}

.dv__scroller {
  box-sizing: border-box;
  height: 128px;
  overflow-y: auto;
  scrollbar-width: none;
  display: grid;
  gap: 12px;
  align-content: start;
  padding: 12px;
  border-radius: 16px;
  background: #231f28;
}

.dv__scroller::-webkit-scrollbar {
  display: none;
}

.dv__scroller:focus-visible {
  outline: 3px solid #ffffff;
  outline-offset: 3px;
}

.dv__block {
  box-sizing: border-box;
  height: 40px;
  padding: 0 16px;
  display: flex;
  align-items: center;
  border-radius: 12px;
  background: rgba(185, 160, 255, 0.22);
  color: #ffffff;
  font-size: 14px;
  font-weight: 700;
  line-height: 20px;
}

.dv__block--tall {
  height: 72px;
}

.dv__line {
  flex: none;
  height: 10px;
  background: #4cd4a6;
  clip-path: inset(0 50% 0 50% round 6px);
}

.dv__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(255, 255, 255, 0.78);
}

.dv__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.dv__meta b {
  min-width: 3ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
  color: #ffffff;
}

@media (prefers-reduced-motion: reduce) {
  .dv__line {
    transition: none;
  }
}

(function () {
  // 기법: scroll 이벤트마다 선의 위치를 getBoundingClientRect 로 재서 진행도를 구하고, 그 값만큼 clip-path 를 양쪽에서 연다.
  // IntersectionObserver 는 쓰지 않는다. 진행도 = 선이 영역 바닥을 지나 올라온 거리 / DRAW_RANGE_PX 이다.
  var DRAW_RANGE_PX = 80;
  var DRAW_MIN_PCT = 0;
  var CYCLE_MS = 2000;

  var root = document.getElementById('dvRoot');
  var scroller = document.getElementById('dvScroller');
  var line = document.getElementById('dvLine');
  var pctEl = document.getElementById('dvPct');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function update() {
    var box = scroller.getBoundingClientRect();
    var lineBox = line.getBoundingClientRect();
    var risen = box.bottom - lineBox.top;
    var p = Math.min(1, Math.max(0, risen / DRAW_RANGE_PX));
    var pct = DRAW_MIN_PCT + (100 - DRAW_MIN_PCT) * p;
    var edge = (100 - pct) / 2;
    line.style.clipPath = 'inset(0 ' + edge.toFixed(2) + '% 0 ' + edge.toFixed(2) + '% round 6px)';
    pctEl.textContent = String(Math.round(pct));
  }
  function stopDemo() { root.classList.remove('is-demo'); }

  scroller.addEventListener('scroll', update);
  scroller.addEventListener('wheel', stopDemo, { passive: true });
  scroller.addEventListener('pointerdown', stopDemo);
  scroller.addEventListener('keydown', stopDemo);

  var t0 = null;
  function tick(ts) {
    if (t0 === null) t0 = ts;
    if (root.classList.contains('is-demo')) {
      var max = scroller.scrollHeight - scroller.clientHeight;
      var phase = ((ts - t0) % CYCLE_MS) / CYCLE_MS;
      var tri = phase < 0.5 ? phase * 2 : 2 - phase * 2;
      scroller.scrollTop = max * tri;
    }
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    scroller.scrollTop = (scroller.scrollHeight - scroller.clientHeight) / 2;
    update();
  } else {
    update();
    requestAnimationFrame(tick);
  }
})();

// ScrollDrawDivider — vanilla 판(03-divl-scroll-draw-from-center) 의 React 포트.
// scroll 이벤트마다 선의 위치를 getBoundingClientRect 로 재서 진행도를 구하고 clip-path 를 양쪽에서 연다.
import { useEffect, useRef } from "react";
import styles from "./ScrollDrawDivider.module.scss";

type Box<T> = { current: T };

const DRAW_RANGE_PX = 80;
const DRAW_MIN_PCT = 0;
const CYCLE_MS = 2000;

export default function ScrollDrawDivider() {
  const scrollerRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement | null>;
  const lineRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement | null>;
  const pctRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const scroller = scrollerRef.current;
    const line = lineRef.current;
    const pctEl = pctRef.current;
    if (!scroller || !line || !pctEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let t0: number | null = null;
    let raf = 0;

    const update = (): void => {
      const box = scroller.getBoundingClientRect();
      const lineBox = line.getBoundingClientRect();
      const risen = box.bottom - lineBox.top;
      const p = Math.min(1, Math.max(0, risen / DRAW_RANGE_PX));
      const pct = DRAW_MIN_PCT + (100 - DRAW_MIN_PCT) * p;
      const edge = (100 - pct) / 2;
      line.style.clipPath = "inset(0 " + edge.toFixed(2) + "% 0 " + edge.toFixed(2) + "% round 6px)";
      pctEl.textContent = String(Math.round(pct));
    };
    const stopDemo = (): void => { demo = false; };
    scroller.addEventListener("scroll", update);
    scroller.addEventListener("wheel", stopDemo, { passive: true });
    scroller.addEventListener("pointerdown", stopDemo);
    scroller.addEventListener("keydown", stopDemo);

    const tick = (ts: number): void => {
      if (t0 === null) t0 = ts;
      if (demo) {
        const max = scroller.scrollHeight - scroller.clientHeight;
        const phase = ((ts - t0) % CYCLE_MS) / CYCLE_MS;
        const tri = phase < 0.5 ? phase * 2 : 2 - phase * 2;
        scroller.scrollTop = max * tri;
      }
      raf = requestAnimationFrame(tick);
    };

    if (reduce) {
      scroller.scrollTop = (scroller.scrollHeight - scroller.clientHeight) / 2;
      update();
    } else {
      update();
      raf = requestAnimationFrame(tick);
    }
    return () => {
      cancelAnimationFrame(raf);
      scroller.removeEventListener("scroll", update);
      scroller.removeEventListener("wheel", stopDemo);
      scroller.removeEventListener("pointerdown", stopDemo);
      scroller.removeEventListener("keydown", stopDemo);
    };
  }, []);

  return (
    <div className={styles.dv}>
      <div className={styles.scroller} ref={scrollerRef} tabIndex={0} aria-label="스크롤 영역">
        <div className={styles.block}>위 단락</div>
        <div className={styles.blockTall}>위 단락</div>
        <div className={styles.line} ref={lineRef} role="separator" />
        <div className={styles.block}>아래 단락</div>
        <div className={styles.blockTall}>아래 단락</div>
      </div>
      <p className={styles.meta}>
        <span className={styles.tag}>DRAW</span> <b ref={pctRef}>0</b>%
      </p>
    </div>
  );
}