09二つのセクションが波の境目で受け渡される区切り線の場面

<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">
    <section class="dv__sec dv__sec--a">
      <p class="dv__title">最初のセクション</p>
    </section>
    <svg class="dv__wave" id="dvWave" viewBox="0 0 320 72" preserveAspectRatio="none" aria-hidden="true" focusable="false">
      <path class="dv__fill" id="dvFill" d="M0 0H320V36H0Z"></path>
      <path class="dv__edge" id="dvEdge" d="M0 36H320" pathLength="1"></path>
    </svg>
    <section class="dv__sec dv__sec--b">
      <div class="dv__bleed" id="dvBleed"></div>
      <p class="dv__title">次のセクション</p>
    </section>
  </div>
  <p class="dv__meta"><span class="dv__tag">PHASE</span> <b id="dvPhase">0</b><span>rad</span></p>
</div>
</main>

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

// 핵심: scrollTop 하나가 파도 위상, 경계선이 그려진 길이, 위 섹션 색이 번진 높이를 한꺼번에 정한다.

$sec-a: $subject-pink;
$sec-b: $subject-sky;

@include stage($stage-ink);

.dv {
  width: min(100%, 360px);
  display: grid;
  gap: $sp-2;
}

.dv__scroller {
  box-sizing: border-box;
  height: 144px;
  overflow-y: auto;
  scrollbar-width: none;
  border-radius: $r-card;
  background: $sec-b;
}
.dv__scroller::-webkit-scrollbar { display: none; }
.dv__scroller:focus-visible { outline: 3px solid $stage-paper; outline-offset: 3px; }

.dv__sec {
  position: relative;
  box-sizing: border-box;
  display: grid;
  align-items: center;
  padding: 0 $sp-4;
}
.dv__sec--a { height: 64px; background: $sec-a; }
.dv__sec--b { height: 136px; background: $sec-b; overflow: hidden; }

.dv__title {
  position: relative;
  margin: 0;
  color: $subject-ink;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
}

.dv__wave {
  display: block;
  width: 100%;
  height: 56px;
  background: $sec-b;
}
.dv__fill { fill: $sec-a; }
.dv__edge {
  fill: none;
  stroke: $stage-paper;
  stroke-width: 3;
  stroke-linecap: round;
  vector-effect: non-scaling-stroke;
}

.dv__bleed {
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  height: 0;
  background: linear-gradient(180deg, $sec-a, rgba($sec-a, 0));
}

.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__bleed { 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: 8px;
}

.dv__scroller {
  box-sizing: border-box;
  height: 144px;
  overflow-y: auto;
  scrollbar-width: none;
  border-radius: 16px;
  background: #6ec8ff;
}

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

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

.dv__sec {
  position: relative;
  box-sizing: border-box;
  display: grid;
  align-items: center;
  padding: 0 16px;
}

.dv__sec--a {
  height: 64px;
  background: #ff9ebb;
}

.dv__sec--b {
  height: 136px;
  background: #6ec8ff;
  overflow: hidden;
}

.dv__title {
  position: relative;
  margin: 0;
  color: #17141a;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
}

.dv__wave {
  display: block;
  width: 100%;
  height: 56px;
  background: #6ec8ff;
}

.dv__fill {
  fill: #ff9ebb;
}

.dv__edge {
  fill: none;
  stroke: #ffffff;
  stroke-width: 3;
  stroke-linecap: round;
  vector-effect: non-scaling-stroke;
}

.dv__bleed {
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  height: 0;
  background: linear-gradient(180deg, #ff9ebb, rgba(255, 158, 187, 0));
}

.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__bleed {
    transition: none;
  }
}

(function () {
  // 기법: 스크롤 위치 scrollTop 이 파도 위상(scrollTop * WAVE_PHASE_PER_PX)을 밀고, 같은 값의 비율이 경계선을 그려 나간다(stroke-dashoffset).
  // 위 섹션 색은 아래 섹션 높이의 BLEED_PCT% 까지 투명해지며 번지고, 번지는 정도도 같은 진행도(0~1)에서 나온다.
  var WAVE_PHASE_PER_PX = 0.07;
  var BLEED_PCT = 60;
  var SCROLL_SHARE = 0.62;
  var DRAW_RANGE_PX = 90;
  var WAVE_AMP = [11, 6];
  var WAVE_PERIOD = [150, 61];
  var WAVE_SPEED = [1, 1.6];
  var BASE_Y = 34;
  var VIEW_W = 320;
  var STEP_PX = 4;
  var CYCLE_MS = 2000;

  var root = document.getElementById('dvRoot');
  var scroller = document.getElementById('dvScroller');
  var wave = document.getElementById('dvWave');
  var fill = document.getElementById('dvFill');
  var edge = document.getElementById('dvEdge');
  var bleed = document.getElementById('dvBleed');
  var phaseEl = document.getElementById('dvPhase');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function edgeY(x, phase) {
    var y = BASE_Y;
    for (var i = 0; i < WAVE_AMP.length; i += 1) {
      y += WAVE_AMP[i] * Math.sin(2 * Math.PI * x / WAVE_PERIOD[i] + phase * WAVE_SPEED[i]);
    }
    return y;
  }
  function pathOf(phase) {
    var pts = [];
    for (var x = 0; x <= VIEW_W; x += STEP_PX) pts.push(x + ' ' + edgeY(x, phase).toFixed(1));
    return pts;
  }
  function update() {
    var top = scroller.scrollTop;
    var phase = top * WAVE_PHASE_PER_PX;
    var pts = pathOf(phase);
    fill.setAttribute('d', 'M0 0H' + VIEW_W + 'V' + pts[pts.length - 1].split(' ')[1] + 'L' + pts.slice().reverse().join('L') + 'Z');
    var line = 'M' + pts.join('L');
    edge.setAttribute('d', line);
    var drawn = Math.min(1, Math.max(0, top / DRAW_RANGE_PX));
    edge.style.strokeDasharray = '1';
    edge.style.strokeDashoffset = String(1 - drawn);
    bleed.style.height = (BLEED_PCT * drawn).toFixed(1) + '%';
    phaseEl.textContent = phase.toFixed(1);
  }
  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 * SCROLL_SHARE * tri;
    }
    requestAnimationFrame(tick);
  }

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

// WaveHandoff — vanilla 판(09-divl-section-handoff-wave-scene) 의 React 포트.
// scrollTop 하나가 파도 위상, 경계선이 그려진 길이, 위 섹션 색이 번진 높이를 한꺼번에 정한다.
import { useEffect, useRef } from "react";
import styles from "./WaveHandoff.module.scss";

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

const WAVE_PHASE_PER_PX = 0.07;
const BLEED_PCT = 60;
const SCROLL_SHARE = 0.62;
const DRAW_RANGE_PX = 90;
const WAVE_AMP: number[] = [11, 6];
const WAVE_PERIOD: number[] = [150, 61];
const WAVE_SPEED: number[] = [1, 1.6];
const BASE_Y = 34;
const VIEW_W = 320;
const STEP_PX = 4;
const CYCLE_MS = 2000;

function edgeY(x: number, phase: number): number {
  let y = BASE_Y;
  for (let i = 0; i < WAVE_AMP.length; i += 1) {
    y += WAVE_AMP[i] * Math.sin((2 * Math.PI * x) / WAVE_PERIOD[i] + phase * WAVE_SPEED[i]);
  }
  return y;
}

export default function WaveHandoff() {
  const scrollerRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement | null>;
  const fillRef = useRef<SVGPathElement | null>(null) as Box<SVGPathElement | null>;
  const edgeRef = useRef<SVGPathElement | null>(null) as Box<SVGPathElement | null>;
  const bleedRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement | null>;
  const phaseRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const scroller = scrollerRef.current;
    const fill = fillRef.current;
    const edge = edgeRef.current;
    const bleed = bleedRef.current;
    const phaseEl = phaseRef.current;
    if (!scroller || !fill || !edge || !bleed || !phaseEl) 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 top = scroller.scrollTop;
      const phase = top * WAVE_PHASE_PER_PX;
      const pts: string[] = [];
      let lastY = BASE_Y;
      for (let x = 0; x <= VIEW_W; x += STEP_PX) {
        lastY = edgeY(x, phase);
        pts.push(x + " " + lastY.toFixed(1));
      }
      fill.setAttribute("d", "M0 0H" + VIEW_W + "V" + lastY.toFixed(1) + "L" + pts.slice().reverse().join("L") + "Z");
      edge.setAttribute("d", "M" + pts.join("L"));
      const drawn = Math.min(1, Math.max(0, top / DRAW_RANGE_PX));
      edge.style.strokeDasharray = "1";
      edge.style.strokeDashoffset = String(1 - drawn);
      bleed.style.height = (BLEED_PCT * drawn).toFixed(1) + "%";
      phaseEl.textContent = phase.toFixed(1);
    };
    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 * SCROLL_SHARE * tri;
      }
      raf = requestAnimationFrame(tick);
    };

    if (reduce) {
      scroller.scrollTop = ((scroller.scrollHeight - scroller.clientHeight) * SCROLL_SHARE) / 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">
        <section className={styles.secA}>
          <p className={styles.title}>첫 섹션</p>
        </section>
        <svg className={styles.wave} viewBox="0 0 320 72" preserveAspectRatio="none" aria-hidden="true" focusable="false">
          <path className={styles.fill} ref={fillRef} d="M0 0H320V36H0Z" />
          <path className={styles.edge} ref={edgeRef} d="M0 36H320" pathLength={1} />
        </svg>
        <section className={styles.secB}>
          <div className={styles.bleed} ref={bleedRef} />
          <p className={styles.title}>다음 섹션</p>
        </section>
      </div>
      <p className={styles.meta}>
        <span className={styles.tag}>PHASE</span> <b ref={phaseRef}>0</b>
        <span>rad</span>
      </p>
    </div>
  );
}