08Gradient bar you build by dragging stops

<main class="stage">
  <div class="gs is-demo">
    <div class="gs__bar" id="gsBar">
      <span class="gs__stop gs__stop--a" aria-hidden="true"></span>
      <span class="gs__stop gs__stop--m" id="gsMid" role="slider" tabindex="0"
            aria-label="Middle stop position"
            aria-valuemin="0" aria-valuemax="100" aria-valuenow="45"
            aria-valuetext="Middle stop at 45%"></span>
      <span class="gs__stop gs__stop--b" aria-hidden="true"></span>
    </div>
    <code class="gs__code" aria-hidden="true"></code>
    <div class="gs__foot">
      <span class="gs__pos" aria-hidden="true">Middle <b class="gs__pct"></b></span>
      <button type="button" class="gs__copy" id="gsCopy" aria-label="Copy the CSS string">
        <svg viewBox="0 0 256 256" fill="currentColor" aria-hidden="true"><path d="M216,40V168H168V88H88V40Z" opacity="0.2"/><path d="M216,32H88a8,8,0,0,0-8,8V80H40a8,8,0,0,0-8,8V216a8,8,0,0,0,8,8H168a8,8,0,0,0,8-8V176h40a8,8,0,0,0,8-8V40A8,8,0,0,0,216,32ZM160,208H48V96H160Zm48-48H176V88a8,8,0,0,0-8-8H96V48H208Z"/></svg>
      </button>
    </div>
    <span class="gs__live" aria-live="polite"></span>
  </div>
</main>

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

// 그라디언트 빌더 — 움직이는 값은 가운데 정지점 위치 --p 하나뿐이다.
// 막대의 색 분포(linear-gradient 의 둘째 정지점), 손잡이 자리(translateX), 아래 CSS 문자열, 퍼센트 숫자가
// 전부 그 한 값에서 나온다. 값이 하나니 넷이 어긋날 수가 없다.
// 264px 트랙에서 1% = 2.64px 이라 손잡이는 translateX(calc(var(--p) * 2.64px)) 로 그 자리에 선다.
// 루프는 45% → 78% → 같은 길을 되짚어 45% 로 돌아온다(텔레포트 없음).
$duration: $dur-loop;
$easing: $ease-spring;
$color: $subject-blue;

@include stage($stage-yellow);

$track: 264px;

@property --p { syntax: "<integer>"; inherits: true; initial-value: 45; }

.gs {
  position: relative;
  box-sizing: border-box;
  width: 288px;
  max-width: 100%;
  padding: $sp-3;
  border-radius: $r-card;
  background: $subject-cream;
  color: $subject-ink;
  font-family: inherit;
}
.gs * { box-sizing: border-box; }

.gs__bar {
  position: relative;
  height: 36px;
  border-radius: $r-sm;
  cursor: pointer;
  touch-action: none;
  background-image: linear-gradient(90deg, $stage-orange 0%, $stage-yellow calc(var(--p) * 1%), $subject-blue 100%);
}

.gs__stop {
  position: absolute;
  top: 50%;
  left: 0;
  width: 16px;
  height: 16px;
  border-radius: 50%;
  border: 3px solid #ffffff;
  box-shadow: $shadow-press;
}
// 손잡이는 자기 중심이 그 퍼센트 위에 놓인다 — 좌표 이동 뒤 자기 크기의 절반만큼 되민다
.gs__stop--a {
  background: $stage-orange;
  transform: translateX(0) translate(-50%, -50%);
}
.gs__stop--m {
  background: $stage-yellow;
  cursor: ew-resize;
  touch-action: none;
  transform: translateX(calc(var(--p) * 2.64px)) translate(-50%, -50%);
}
.gs__stop--b {
  background: $subject-blue;
  transform: translateX($track) translate(-50%, -50%);
}
.gs__stop--m:focus-visible { outline: 3px solid $color; outline-offset: 3px; }

// 아래 CSS 문자열 — 퍼센트 자리만 계수기가 채운다. 색 이름은 sass 가 토큰 값을 그대로 박아 넣는다
.gs__code {
  display: block;
  margin-top: $sp-2;
  counter-set: cp calc(var(--p));
  word-break: break-all;
  font-size: 10px;
  font-weight: 500;
  line-height: 1.4;
  color: rgba(23, 20, 26, .74);
}
.gs__code::after {
  content: "linear-gradient(90deg, #{$stage-orange} 0%, #{$stage-yellow} " counter(cp) "%, #{$subject-blue} 100%)";
}

.gs__foot {
  display: flex;
  align-items: center;
  gap: $sp-2;
  margin-top: $sp-2;
}
.gs__pos {
  font-size: 11px;
  font-weight: 700;
  line-height: 1.2;
}
.gs__pct { counter-set: cp calc(var(--p)); font-weight: 800; }
.gs__pct::after { content: counter(cp) "%"; }

.gs__copy {
  margin-left: auto;
  display: grid;
  place-items: center;
  width: 28px;
  height: 28px;
  padding: 0;
  border: 2px solid rgba(23, 20, 26, .16);
  border-radius: $r-control;
  background: transparent;
  color: inherit;
  cursor: pointer;
}
.gs__copy svg { width: 16px; height: 16px; }
.gs__copy:focus-visible { outline: 3px solid $color; outline-offset: 2px; }

// 화면에는 안 보이되 보조기술에는 읽히는 알림 줄
.gs__live {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

// ── 미리보기 루프(정적 규칙 먼저, 그 위에 덧씌움) ─────────────────
// 애니메이션하는 것은 --p 하나뿐이다. 막대·손잡이·문자열·숫자가 같은 값을 읽으니
// 어느 프레임에서도 넷이 같은 순간을 가리킨다
.gs.is-demo {
  animation-name: gsSlide;
  animation-duration: $duration;
  animation-timing-function: $easing;
  animation-iteration-count: infinite;
}

@keyframes gsSlide {
  0%        { --p: 45; }
  45%       { --p: 78; }
  90%, 100% { --p: 45; }
}

@media (prefers-reduced-motion: reduce) {
  .gs.is-demo {
    animation: none !important;
  }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

@property --p {
  syntax: "<integer>";
  inherits: true;
  initial-value: 45;
}
.gs {
  position: relative;
  box-sizing: border-box;
  width: 288px;
  max-width: 100%;
  padding: 12px;
  border-radius: 16px;
  background: #fff7e6;
  color: #17141a;
  font-family: inherit;
}

.gs * {
  box-sizing: border-box;
}

.gs__bar {
  position: relative;
  height: 36px;
  border-radius: 8px;
  cursor: pointer;
  touch-action: none;
  background-image: linear-gradient(90deg, #ff4d1f 0%, #ffd23f calc(var(--p) * 1%), #2f6df6 100%);
}

.gs__stop {
  position: absolute;
  top: 50%;
  left: 0;
  width: 16px;
  height: 16px;
  border-radius: 50%;
  border: 3px solid #ffffff;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
}

.gs__stop--a {
  background: #ff4d1f;
  transform: translateX(0) translate(-50%, -50%);
}

.gs__stop--m {
  background: #ffd23f;
  cursor: ew-resize;
  touch-action: none;
  transform: translateX(calc(var(--p) * 2.64px)) translate(-50%, -50%);
}

.gs__stop--b {
  background: #2f6df6;
  transform: translateX(264px) translate(-50%, -50%);
}

.gs__stop--m:focus-visible {
  outline: 3px solid #2f6df6;
  outline-offset: 3px;
}

.gs__code {
  display: block;
  margin-top: 8px;
  counter-set: cp calc(var(--p));
  word-break: break-all;
  font-size: 10px;
  font-weight: 500;
  line-height: 1.4;
  color: rgba(23, 20, 26, 0.74);
}

.gs__code::after {
  content: "linear-gradient(90deg, #ff4d1f 0%, #ffd23f " counter(cp) "%, #2f6df6 100%)";
}

.gs__foot {
  display: flex;
  align-items: center;
  gap: 8px;
  margin-top: 8px;
}

.gs__pos {
  font-size: 11px;
  font-weight: 700;
  line-height: 1.2;
}

.gs__pct {
  counter-set: cp calc(var(--p));
  font-weight: 800;
}

.gs__pct::after {
  content: counter(cp) "%";
}

.gs__copy {
  margin-left: auto;
  display: grid;
  place-items: center;
  width: 28px;
  height: 28px;
  padding: 0;
  border: 2px solid rgba(23, 20, 26, 0.16);
  border-radius: 12px;
  background: transparent;
  color: inherit;
  cursor: pointer;
}

.gs__copy svg {
  width: 16px;
  height: 16px;
}

.gs__copy:focus-visible {
  outline: 3px solid #2f6df6;
  outline-offset: 2px;
}

.gs__live {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

.gs.is-demo {
  animation-name: gsSlide;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite;
}

@keyframes gsSlide {
  0% {
    --p: 45;
  }
  45% {
    --p: 78;
  }
  90%, 100% {
    --p: 45;
  }
}
@media (prefers-reduced-motion: reduce) {
  .gs.is-demo {
    animation: none !important;
  }
}

(function () {
  var root = document.querySelector('.gs');
  var bar = document.getElementById('gsBar');
  var mid = document.getElementById('gsMid');
  var copyBtn = document.getElementById('gsCopy');
  var live = root.querySelector('.gs__live');
  var midFormat = 'Middle stop at %p%';
  var copiedText = 'Copied';
  var pos = 45;

  // 화면에 보이는 문자열과 복사되는 문자열은 같은 pos 하나에서 나온다.
  // 막대 배경·손잡이 자리·아래 글자는 CSS 가 --p 에서 만들고, 복사본만 여기서 조립한다
  function cssText() {
    return 'linear-gradient(90deg, #ff4d1f 0%, #ffd23f ' + pos + '%, #2f6df6 100%)';
  }
  function apply() {
    root.style.setProperty('--p', pos);
    mid.setAttribute('aria-valuenow', pos);
    mid.setAttribute('aria-valuetext', midFormat.replace('%p', pos));
  }
  function pickAt(e) {
    var box = bar.getBoundingClientRect();
    pos = Math.round(Math.min(100, Math.max(0, (e.clientX - box.left) / box.width * 100)));
    apply();
  }

  bar.addEventListener('pointerdown', function (e) {
    root.classList.remove('is-demo');
    bar.setPointerCapture(e.pointerId);   // 막대 밖으로 끌어도 이 요소가 계속 포인터를 받는다
    pickAt(e);
  });
  bar.addEventListener('pointermove', function (e) {
    if (bar.hasPointerCapture(e.pointerId)) { pickAt(e); }
  });

  mid.addEventListener('keydown', function (e) {
    var step = e.shiftKey ? 10 : 1;
    if (e.key === 'ArrowRight') { pos = Math.min(100, pos + step); }
    else if (e.key === 'ArrowLeft') { pos = Math.max(0, pos - step); }
    else { return; }
    e.preventDefault();
    root.classList.remove('is-demo');
    apply();
  });

  copyBtn.addEventListener('click', function () {
    root.classList.remove('is-demo');
    if (navigator.clipboard) { navigator.clipboard.writeText(cssText()); }
    live.textContent = copiedText;
  });

  apply();
})();

// GradientStops — vanilla 판(run/286/demo/08-color-picker-gradient-stops) 의 React 포트.
// 변수 3개(duration·easing·color)는 props 로 받아 CSS 변수(var(--duration) 등)로 들어온다.
// 상태는 가운데 정지점 위치 하나(0~100)뿐이다. 막대의 색 분포도, 손잡이 자리도, 아래 CSS 문자열도,
// 퍼센트 숫자도 전부 그 한 값(--p)에서 나오므로 넷이 어긋날 수가 없다.
import { useRef, useState } from "react";
import type { CSSProperties, PointerEvent as ReactPointerEvent } from "react";
import styles from "./GradientStops.module.scss";

type KeyEvt = { key: string; shiftKey: boolean; preventDefault(): void };

export type GradientStopsProps = {
  midLabel?: string;
  /** "가운데 정지점 %p%" 처럼 자리표시자 하나를 가진 틀 */
  midFormat?: string;
  posLabel?: string;
  copyLabel?: string;
  copiedText?: 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;
  "--p"?: number;
};

function clampPercent(n: number): number {
  return Math.round(Math.min(100, Math.max(0, n)));
}

export default function GradientStops({
  midLabel = "가운데 정지점 위치",
  midFormat = "가운데 정지점 %p%",
  posLabel = "가운데",
  copyLabel = "CSS 문자열 복사",
  copiedText = "복사했습니다",
  demo = true,
  duration = 2000,
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#2f6df6",
}: GradientStopsProps) {
  const [live, setLive] = useState(demo);
  const [pos, setPos] = useState(45);
  const [notice, setNotice] = useState("");
  const barRef = useRef<HTMLDivElement>(null);

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

  // 화면에 보이는 문자열과 복사되는 문자열은 같은 pos 하나에서 나온다
  function cssText(): string {
    return `linear-gradient(90deg, #ff4d1f 0%, #ffd23f ${pos}%, #2f6df6 100%)`;
  }

  function pickAt(e: ReactPointerEvent<HTMLDivElement>) {
    const node = barRef.current;
    if (!node) return;
    const box = node.getBoundingClientRect();
    setPos(clampPercent(((e.clientX - box.left) / box.width) * 100));
  }

  function start(e: ReactPointerEvent<HTMLDivElement>) {
    setLive(false);
    // 막대 밖으로 끌어도 이 요소가 계속 포인터를 받는다
    e.currentTarget.setPointerCapture(e.pointerId);
    pickAt(e);
  }

  function drag(e: ReactPointerEvent<HTMLDivElement>) {
    if (e.currentTarget.hasPointerCapture(e.pointerId)) pickAt(e);
  }

  function keyMid(e: KeyEvt) {
    const step = e.shiftKey ? 10 : 1;
    if (e.key === "ArrowRight") setPos(Math.min(100, pos + step));
    else if (e.key === "ArrowLeft") setPos(Math.max(0, pos - step));
    else return;
    e.preventDefault();
    setLive(false);
  }

  function copy() {
    setLive(false);
    if (navigator.clipboard) navigator.clipboard.writeText(cssText());
    setNotice(copiedText);
  }

  return (
    <div className={[styles.gs, live ? styles.isDemo : ""].filter(Boolean).join(" ")} style={vars}>
      <div ref={barRef} className={styles.bar} onPointerDown={start} onPointerMove={drag}>
        <span className={[styles.stop, styles.stopA].join(" ")} aria-hidden="true" />
        <span
          className={[styles.stop, styles.stopM].join(" ")}
          role="slider"
          tabIndex={0}
          aria-label={midLabel}
          aria-valuemin={0}
          aria-valuemax={100}
          aria-valuenow={pos}
          aria-valuetext={midFormat.replace("%p", String(pos))}
          onKeyDown={keyMid}
        />
        <span className={[styles.stop, styles.stopB].join(" ")} aria-hidden="true" />
      </div>
      <code className={styles.code} aria-hidden="true" />
      <div className={styles.foot}>
        <span className={styles.pos} aria-hidden="true">
          {posLabel} <b className={styles.pct} />
        </span>
        <button type="button" className={styles.copy} aria-label={copyLabel} onClick={copy}>
          <svg viewBox="0 0 256 256" fill="currentColor" aria-hidden="true">
            <path d="M216,40V168H168V88H88V40Z" opacity="0.2" />
            <path d="M216,32H88a8,8,0,0,0-8,8V80H40a8,8,0,0,0-8,8V216a8,8,0,0,0,8,8H168a8,8,0,0,0,8-8V176h40a8,8,0,0,0,8-8V40A8,8,0,0,0,216,32ZM160,208H48V96H160Zm48-48H176V88a8,8,0,0,0-8-8H96V48H208Z" />
          </svg>
        </button>
      </div>
      <span className={styles.liveRegion} aria-live="polite">
        {notice}
      </span>
    </div>
  );
}