07Velocity response

<main class="stage">
  <section class="vl is-demo" id="vl" aria-label="Velocity response demo">
    <header class="vl__head">
      <p class="vl__title">Velocity response</p>
      <p class="vl__attr"><code>--vel</code></p>
    </header>
    <div class="vl__view" id="vl-view" tabindex="0" role="region" aria-label="Title preview — try scrolling fast">
      <div class="vl__track">
        <div class="vl__rule" aria-hidden="true"><span></span><span></span></div>
        <h2 class="vl__line">Scroll fast and</h2>
        <h2 class="vl__line">the type leans</h2>
        <h2 class="vl__line">and spreads</h2>
        <p class="vl__filler" aria-hidden="true">Next section</p>
      </div>
    </div>
    <footer class="vl__meter" aria-label="Scroll speed">
      <span class="vl__bar"><span class="vl__fill" id="vl-fill"></span></span>
      <b class="vl__num" id="vl-num">0.00</b>
    </footer>
  </section>
</main>

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

// 속도 반응 — 스크롤 속도를 --vel 로 넘겨 글자가 skewY 로 눕고 줄이 벌어진다
// 각도는 clamp() 로 ±6deg 안에 묶인다. 속도 0 이면 제자리. @property 등록이 있어야 값이 보간된다
$duration: $dur-loop;
$easing: $ease-spring;
$color: $subject-blue;

@property --vel {
  syntax: "<number>";
  inherits: true;
  initial-value: 0;
}

@include stage($stage-paper);

.vl {
  position: relative;
  box-sizing: border-box;
  width: min(312px, 92vw);
  border-radius: $r-card;
  background: $subject-cream;
  color: $subject-ink;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  overflow: hidden;
  box-shadow: $shadow-raised;
}

.vl__head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: $sp-2;
  padding: $sp-2 $sp-3;
}
.vl__title {
  margin: 0;
  font-size: 13px;
  font-weight: 800;
  line-height: 1.3;
}
.vl__attr {
  margin: 0;
  font-size: 10px;
  font-weight: 400;
  line-height: 1.3;
  color: rgba($subject-ink, .72);
  code { font-family: "Consolas", "Menlo", monospace; }
}

.vl__view {
  height: 150px;
  overflow-y: auto;
  padding: 0 $sp-3;
  outline: none;
}
.vl__track {
  display: grid;
  gap: $sp-3;
  padding-top: $sp-2;
  padding-bottom: 140px;
}

// 윗세기 — 타이틀 위의 쉬어가는 자리 (텍스트 없는 장식 규칙 두 줄)
.vl__rule {
  display: grid;
  gap: $sp-1;
  span {
    height: 8px;
    border-radius: $r-pill;
    background: rgba($subject-ink, .12);
    &:last-child { width: 64%; }
  }
}

.vl__line {
  margin: 0;
  font-size: 24px;
  font-weight: 800;
  line-height: 1.4;
  // 홀수 줄은 왼쪽으로 눕고 위로 벌어진다, 짝수 줄은 반대 — 각도·벌어짐 모두 clamp() 범위 안
  &:nth-child(odd) {
    transform: skewY(clamp(-6deg, calc(var(--vel, 0) * -7deg), 6deg)) translateY(calc(var(--vel, 0) * -4px));
  }
  &:nth-child(even) {
    transform: skewY(clamp(-6deg, calc(var(--vel, 0) * 7deg), 6deg)) translateY(calc(var(--vel, 0) * 4px));
  }
}

.vl__filler {
  margin: 0;
  font-size: 10px;
  font-weight: 700;
  line-height: 1.3;
  color: rgba($subject-ink, .72);
  border-radius: $r-control;
  background: rgba($subject-ink, .06);
  padding: $sp-6 0;
  text-align: center;
}

.vl__meter {
  display: flex;
  align-items: center;
  gap: $sp-2;
  padding: $sp-2 $sp-3;
  border-top: 1px solid rgba($subject-ink, .12);
}
.vl__bar {
  flex: 1;
  height: 8px;
  border-radius: $r-pill;
  background: rgba($subject-ink, .12);
  overflow: hidden;
}
.vl__fill {
  display: block;
  height: 100%;
  border-radius: $r-pill;
  background: $color;
  transform-origin: left center;
  transform: scaleX(clamp(0, var(--vel, 0), 1));
}
.vl__num {
  font-family: "Consolas", "Menlo", monospace;
  font-size: 12px;
  font-weight: 700;
  line-height: 1;
  min-width: 36px;
  text-align: right;
}

// 미리보기 — --vel 을 등록된 애니메이션으로 물결치게 하고 트랙이 두 번 흘러간다
// 애니메이션 값은 인라인 --vel 보다 우선하므로 미리보기 중 숫자·기울기가 함께 움직인다
.vl.is-demo {
  animation-name: vl-vel;
  animation-duration: $duration;
  animation-timing-function: linear;
  animation-iteration-count: infinite;

  .vl__track {
    animation-name: vl-flow;
    animation-duration: $duration;
    animation-timing-function: $easing;
    animation-iteration-count: infinite;
    animation-fill-mode: backwards;
  }
}

@keyframes vl-vel {
  0%, 5% { --vel: 0; }
  18% { --vel: 1; }
  30%, 38% { --vel: 0.1; }
  55% { --vel: 0.9; }
  68%, 100% { --vel: 0; }
}
@keyframes vl-flow {
  0%, 5% { transform: translateY(0); }
  18%, 40% { transform: translateY(-60px); }
  62%, 100% { transform: translateY(-120px); }
}

@media (max-width: 340px) {
  .vl { width: 100%; }
  .vl__view { height: 104px; }
  .vl__line { font-size: 20px; }
  .vl__filler { padding: $sp-4 0; }
}

@media (prefers-reduced-motion: reduce) {
  .vl.is-demo { animation: none !important; }
  .vl.is-demo .vl__track { animation: none !important; }
  // 정지 상태 — 기울기 없이 글자가 바르게 보인다
  .vl.is-demo .vl__line { transform: none !important; }
  .vl.is-demo .vl__fill { transform: none !important; }
}

@property --vel {
  syntax: "<number>";
  inherits: true;
  initial-value: 0;
}
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);
}

.vl {
  position: relative;
  box-sizing: border-box;
  width: min(312px, 92vw);
  border-radius: 16px;
  background: #fff7e6;
  color: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.vl__head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 8px;
  padding: 8px 12px;
}

.vl__title {
  margin: 0;
  font-size: 13px;
  font-weight: 800;
  line-height: 1.3;
}

.vl__attr {
  margin: 0;
  font-size: 10px;
  font-weight: 400;
  line-height: 1.3;
  color: rgba(23, 20, 26, 0.72);
}
.vl__attr code {
  font-family: "Consolas", "Menlo", monospace;
}

.vl__view {
  height: 150px;
  overflow-y: auto;
  padding: 0 12px;
  outline: none;
}

.vl__track {
  display: grid;
  gap: 12px;
  padding-top: 8px;
  padding-bottom: 140px;
}

.vl__rule {
  display: grid;
  gap: 4px;
}
.vl__rule span {
  height: 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.12);
}
.vl__rule span:last-child {
  width: 64%;
}

.vl__line {
  margin: 0;
  font-size: 24px;
  font-weight: 800;
  line-height: 1.4;
}
.vl__line:nth-child(odd) {
  transform: skewY(clamp(-6deg, var(--vel, 0) * -7deg, 6deg)) translateY(calc(var(--vel, 0) * -4px));
}
.vl__line:nth-child(even) {
  transform: skewY(clamp(-6deg, var(--vel, 0) * 7deg, 6deg)) translateY(calc(var(--vel, 0) * 4px));
}

.vl__filler {
  margin: 0;
  font-size: 10px;
  font-weight: 700;
  line-height: 1.3;
  color: rgba(23, 20, 26, 0.72);
  border-radius: 12px;
  background: rgba(23, 20, 26, 0.06);
  padding: 24px 0;
  text-align: center;
}

.vl__meter {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 8px 12px;
  border-top: 1px solid rgba(23, 20, 26, 0.12);
}

.vl__bar {
  flex: 1;
  height: 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.12);
  overflow: hidden;
}

.vl__fill {
  display: block;
  height: 100%;
  border-radius: 999px;
  background: #2f6df6;
  transform-origin: left center;
  transform: scaleX(clamp(0, var(--vel, 0), 1));
}

.vl__num {
  font-family: "Consolas", "Menlo", monospace;
  font-size: 12px;
  font-weight: 700;
  line-height: 1;
  min-width: 36px;
  text-align: right;
}

.vl.is-demo {
  animation-name: vl-vel;
  animation-duration: 2s;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
}
.vl.is-demo .vl__track {
  animation-name: vl-flow;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite;
  animation-fill-mode: backwards;
}

@keyframes vl-vel {
  0%, 5% {
    --vel: 0;
  }
  18% {
    --vel: 1;
  }
  30%, 38% {
    --vel: 0.1;
  }
  55% {
    --vel: 0.9;
  }
  68%, 100% {
    --vel: 0;
  }
}
@keyframes vl-flow {
  0%, 5% {
    transform: translateY(0);
  }
  18%, 40% {
    transform: translateY(-60px);
  }
  62%, 100% {
    transform: translateY(-120px);
  }
}
@media (max-width: 340px) {
  .vl {
    width: 100%;
  }
  .vl__view {
    height: 104px;
  }
  .vl__line {
    font-size: 20px;
  }
  .vl__filler {
    padding: 16px 0;
  }
}
@media (prefers-reduced-motion: reduce) {
  .vl.is-demo {
    animation: none !important;
  }
  .vl.is-demo .vl__track {
    animation: none !important;
  }
  .vl.is-demo .vl__line {
    transform: none !important;
  }
  .vl.is-demo .vl__fill {
    transform: none !important;
  }
}

// Velocity response — 프레임 사이 이동량으로 스크롤 속도를 재고 --vel 커스텀 속성로 넘긴다
// CSS 는 그 값을 skewY 각도로 쓰고 각도는 clamp() 로 ±6deg 안에 묶인다. 속도가 0 이 되면 제자리로 돌아온다
var root = document.getElementById('vl');
var view = document.getElementById('vl-view');
var num = document.getElementById('vl-num');
var lastY = view.scrollTop;
var vel = 0;
function tick() {
  var y = view.scrollTop;
  var dy = Math.abs(y - lastY);
  lastY = y;
  // 이동량을 0~1 로 정규화해 여유있게 따라간다 — 급변하지 않고 멈추면 감쇠해 0 으로 돌아온다
  var target = Math.min(dy / 26, 1);
  vel += (target - vel) * 0.16;
  if (vel < 0.004) vel = 0;
  root.style.setProperty('--vel', vel.toFixed(4));
  // 읽는 순간의 --vel — 미리보기(.is-demo) 중에는 애니메이션 값이 인라인 값보다 우선한다
  var shown = parseFloat(getComputedStyle(root).getPropertyValue('--vel')) || 0;
  num.textContent = shown.toFixed(2);
  requestAnimationFrame(tick);
}
// reduce 에서는 속도 효과만 끈다 — 엔진을 돌리지 않아 --vel 은 0 그대로다 (기울기 0deg)
if (!matchMedia('(prefers-reduced-motion: reduce)').matches) requestAnimationFrame(tick);
function start() { root.classList.remove('is-demo'); }
view.addEventListener('scroll', start, { passive: true });
view.addEventListener('pointerdown', start);
view.addEventListener('keydown', function (e) {
  if (e.key === 'ArrowDown' || e.key === 'ArrowUp') start();
});

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

export type VelocityResponseProps = {
  title?: string;
  attr?: string;
  lines?: string[];
  filler?: string;
  meterLabel?: string;
  groupLabel?: string;
  listLabel?: string;
  /** 격자 미리보기용 2초 자동 루프. 사람이 손대면 꺼진다 */
  demo?: boolean;
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
// ambient 가 얇아 DOM 요소 타입을 못 쓴다 — 필요한 꼴만 로컬 타입으로 선언
type Box<T> = { current: T };
type KeyEvt = { key: string };

const DEFAULT_LINES: string[] = ["빠르게 내리면", "글자가 눕고", "벌어진다"];

export default function VelocityResponse({
  title = "속도 반응",
  attr = "--vel",
  lines = DEFAULT_LINES,
  filler = "다음 섹션",
  meterLabel = "스크롤 속도",
  groupLabel = "속도 반응 데모",
  listLabel = "타이틀 미리보기 — 빠르게 스크롤해 보세요",
  demo = true,
  duration = 2000,
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#2f6df6",
}: VelocityResponseProps) {
  const [live, setLive] = useState(demo);
  const [num, setNum] = useState("0.00");
  const rootRef = useRef<HTMLElement>(null) as unknown as Box<HTMLElement | null>;
  const viewRef = useRef<HTMLDivElement>(null) as unknown as Box<HTMLDivElement | null>;

  useEffect(() => {
    const root = rootRef.current;
    const view = viewRef.current;
    if (!root || !view) return;
    let lastY = view.scrollTop;
    let vel = 0;
    let raf = 0;
    // 프레임 사이 이동량으로 스크롤 속도를 재고 --vel 로 넘긴다 — 멈추면 감쇠해 0 으로 돌아온다
    const tick = () => {
      const y = view.scrollTop;
      const dy = Math.abs(y - lastY);
      lastY = y;
      const target = Math.min(dy / 26, 1);
      vel += (target - vel) * 0.16;
      if (vel < 0.004) vel = 0;
      root.style.setProperty("--vel", vel.toFixed(4));
      // 읽는 순간의 --vel — 미리보기 중에는 애니메이션 값이 인라인 값보다 우선한다
      const shown = parseFloat(getComputedStyle(root).getPropertyValue("--vel")) || 0;
      setNum(shown.toFixed(2));
      raf = requestAnimationFrame(tick);
    };
    // reduce 에서는 속도 효과만 끈다 — 엔진을 돌리지 않아 --vel 은 0 그대로다 (기울기 0deg)
    const reduce = matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) raf = requestAnimationFrame(tick);
    return () => cancelAnimationFrame(raf);
  }, []);

  const vars: Vars = {
    "--duration": `${duration}ms`,
    "--easing": easing,
    "--color": color,
  };

  return (
    <section
      ref={rootRef as unknown as Box<HTMLElement>}
      className={`${styles.vl} ${live ? styles.isDemo : ""}`}
      style={vars}
      aria-label={groupLabel}
    >
      <header className={styles.head}>
        <p className={styles.title}>{title}</p>
        <p className={styles.attr}><code>{attr}</code></p>
      </header>
      <div
        className={styles.view}
        ref={viewRef as unknown as Box<HTMLDivElement>}
        onScroll={() => setLive(false)}
        onPointerDown={() => setLive(false)}
        onKeyDown={(e: KeyEvt) => {
          if (e.key === "ArrowDown" || e.key === "ArrowUp") setLive(false);
        }}
        tabIndex={0}
        role="region"
        aria-label={listLabel}
      >
        <div className={styles.track}>
          <div className={styles.rule} aria-hidden="true"><span /><span /></div>
          {lines.map((text, i) => (
            <h2 key={text} className={`${styles.line} ${i % 2 === 0 ? styles.lineOdd : styles.lineEven}`}>
              {text}
            </h2>
          ))}
          <p className={styles.filler} aria-hidden="true">{filler}</p>
        </div>
      </div>
      <footer className={styles.meter} aria-label={meterLabel}>
        <span className={styles.bar}><span className={styles.fill} /></span>
        <b className={styles.num}>{num}</b>
      </footer>
    </section>
  );
}