01Value bubble — converting track position to a percentage the label follows

<main class="stage">
<div class="vt" id="vt">
  <p class="vt__hint">Computing the percentage into a CSS variable makes the tooltip follow the thumb exactly</p>
  <div class="vt__wrap" id="vtWrap" role="img" aria-label="Scene where a tooltip label moves with the thumb every time the value changes">
    <div class="vt__bubble" id="vtBubble"><span id="vtVal">50</span></div>
    <input class="vt__range" id="vtRange" type="range" min="0" max="100" step="1" value="50" aria-label="Value slider">
  </div>
  <p class="vt__meta"><b class="vt__tag" id="vtCalc">50.0%</b><span class="vt__unit">Tooltip position from calc()</span></p>
</div>
</main>

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

// 값 풍선(01) — input[type=range] 의 runnable-track/progress 를 var(--pct) 로 채우고,
// 풍선은 계산된 left(calc) 로 썸을 따라간다. 트랙 두께는 변형 표의 기본값 8px.

$duration: $dur-quick;
$easing: $ease-out;
$color: $subject-blue-deep;

@include stage($stage-ink);

$track-h: 8px;
$thumb-d: 20px;

.vt {
  display: flex; flex-direction: column; align-items: center; gap: $sp-3;
  width: 100%; max-width: 360px;
}

.vt__hint {
  margin: 0; font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); line-height: 1.35;
  color: $subject-cream; text-align: center;
}

.vt__wrap {
  position: relative; width: 100%; height: 48px;
  display: flex; align-items: center;
  --pct: 50;
  --tw: #{$thumb-d};
}

.vt__bubble {
  position: absolute; top: 0;
  left: calc((var(--pct) / 100) * (100% - var(--tw)) + var(--tw) / 2);
  transform: translate(-50%, 0);
  min-width: 32px; padding: $sp-1 $sp-2; box-sizing: border-box;
  background: $color; color: $subject-cream;
  border-radius: $r-sm;
  font-weight: 700; font-size: 11px; line-height: 1.2; text-align: center;
  font-variant-numeric: tabular-nums;
  box-shadow: $shadow-press;
  transition: background $duration $easing;
}

.vt__range {
  appearance: none; -webkit-appearance: none;
  width: 100%; height: $thumb-d; margin: 0; background: transparent;
  position: relative; top: 14px;
}
.vt__range:focus-visible { outline: 2px solid $subject-cream; outline-offset: 4px; border-radius: $r-pill; }

.vt__range::-webkit-slider-runnable-track {
  height: $track-h; border-radius: $r-pill;
  background: linear-gradient(to right, $color calc(var(--pct) * 1%), rgba($subject-cream, .16) calc(var(--pct) * 1%));
}
.vt__range::-webkit-slider-thumb {
  appearance: none; -webkit-appearance: none;
  width: $thumb-d; height: $thumb-d; border-radius: 50%;
  background: $subject-cream; box-shadow: $shadow-press;
  margin-top: calc(($track-h - $thumb-d) / 2);
}
.vt__range::-moz-range-track {
  height: $track-h; border-radius: $r-pill; background: rgba($subject-cream, .16);
}
.vt__range::-moz-range-progress {
  height: $track-h; border-radius: $r-pill; background: $color;
}
.vt__range::-moz-range-thumb {
  width: $thumb-d; height: $thumb-d; border: 0; border-radius: 50%;
  background: $subject-cream; box-shadow: $shadow-press;
}

.vt__meta { margin: 0; font-size: clamp(9px, 2.4vw, 11px); color: rgba($subject-cream, .82); }
.vt__tag { font-weight: 700; color: $stage-yellow; font-variant-numeric: tabular-nums; margin-right: $sp-1; }
.vt__unit { color: rgba($subject-cream, .72); }

@media (max-width: 340px) {
  .vt { gap: $sp-2; max-width: 280px; }
  .vt__hint { font-size: 8px; }
  .vt__meta { font-size: 7px; }
}

@media (prefers-reduced-motion: reduce) {
  .vt__bubble { 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);
}

.vt {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  width: 100%;
  max-width: 360px;
}

.vt__hint {
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  line-height: 1.35;
  color: #fff7e6;
  text-align: center;
}

.vt__wrap {
  position: relative;
  width: 100%;
  height: 48px;
  display: flex;
  align-items: center;
  --pct: 50;
  --tw: 20px;
}

.vt__bubble {
  position: absolute;
  top: 0;
  left: calc(var(--pct) / 100 * (100% - var(--tw)) + var(--tw) / 2);
  transform: translate(-50%, 0);
  min-width: 32px;
  padding: 4px 8px;
  box-sizing: border-box;
  background: #0a4fe8;
  color: #fff7e6;
  border-radius: 8px;
  font-weight: 700;
  font-size: 11px;
  line-height: 1.2;
  text-align: center;
  font-variant-numeric: tabular-nums;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  transition: background 220ms ease-out;
}

.vt__range {
  appearance: none;
  -webkit-appearance: none;
  width: 100%;
  height: 20px;
  margin: 0;
  background: transparent;
  position: relative;
  top: 14px;
}

.vt__range:focus-visible {
  outline: 2px solid #fff7e6;
  outline-offset: 4px;
  border-radius: 999px;
}

.vt__range::-webkit-slider-runnable-track {
  height: 8px;
  border-radius: 999px;
  background: linear-gradient(to right, #0a4fe8 calc(var(--pct) * 1%), rgba(255, 247, 230, 0.16) calc(var(--pct) * 1%));
}

.vt__range::-webkit-slider-thumb {
  appearance: none;
  -webkit-appearance: none;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background: #fff7e6;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  margin-top: -6px;
}

.vt__range::-moz-range-track {
  height: 8px;
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.16);
}

.vt__range::-moz-range-progress {
  height: 8px;
  border-radius: 999px;
  background: #0a4fe8;
}

.vt__range::-moz-range-thumb {
  width: 20px;
  height: 20px;
  border: 0;
  border-radius: 50%;
  background: #fff7e6;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
}

.vt__meta {
  margin: 0;
  font-size: clamp(9px, 2.4vw, 11px);
  color: rgba(255, 247, 230, 0.82);
}

.vt__tag {
  font-weight: 700;
  color: #ffd23f;
  font-variant-numeric: tabular-nums;
  margin-right: 4px;
}

.vt__unit {
  color: rgba(255, 247, 230, 0.72);
}

@media (max-width: 340px) {
  .vt {
    gap: 8px;
    max-width: 280px;
  }
  .vt__hint {
    font-size: 8px;
  }
  .vt__meta {
    font-size: 7px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .vt__bubble {
    transition: none;
  }
}

// 값 풍선(01) — (value - min) / (max - min) * 100 을 --pct 로 내려 풍선의 left 를
// calc(var(--pct) * 1%) 로 계산한다. 풍선 폭의 절반만큼 안쪽으로 당기는 오프셋 보정을
// 넣어 0%·100% 에서도 풍선이 트랙 밖으로 삐져나가지 않게 한다. CSS @keyframes 는 없다 —
// 자동 시연은 목표값 사이를 지수 감쇠로 부드럽게 접근하는 requestAnimationFrame 루프다.
(function () {
  var root = document.getElementById('vt');
  var wrap = document.getElementById('vtWrap');
  var bubble = document.getElementById('vtBubble');
  var valEl = document.getElementById('vtVal');
  var calcEl = document.getElementById('vtCalc');
  var range = document.getElementById('vtRange');
  if (!root || !wrap || !bubble || !valEl || !calcEl || !range) return;

  var MIN = 0, MAX = 100;
  var TAU = 0.11; // 지수 감쇠 시간상수(s) — 값 접근 속도
  var current = 50;

  function paint(v) {
    var pct = (v - MIN) / (MAX - MIN) * 100;
    wrap.style.setProperty('--pct', pct.toFixed(2));
    valEl.textContent = String(Math.round(v));
    calcEl.textContent = pct.toFixed(1) + '%';
    range.setAttribute('aria-valuenow', String(Math.round(v)));
  }
  paint(current);

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function onUserInput() {
    stop();
    current = Number(range.value);
    paint(current);
  }
  range.addEventListener('input', onUserInput);
  range.addEventListener('pointerdown', stop);
  range.addEventListener('keydown', stop);

  var targets = [50, 18, 82, 35, 92, 50]; // 마지막 = 처음, 루프가 이어 붙는다
  var seg = 0, playing = true;

  function stop() { playing = false; }

  if (reduce) {
    current = targets[0];
    range.value = String(current);
    paint(current);
  } else {
    var segStart = performance.now();
    var segFrom = current;
    var SEG_MS = 400;
    (function step(now) {
      if (playing) {
        var elapsed = now - segStart;
        if (elapsed >= SEG_MS) {
          seg = (seg + 1) % targets.length;
          segStart = now;
          segFrom = current;
          elapsed = 0;
        }
        var target = targets[seg];
        var t = elapsed / 1000;
        var k = 1 - Math.exp(-t / TAU);
        current = segFrom + (target - segFrom) * k;
        range.value = String(Math.round(current));
        paint(current);
      }
      requestAnimationFrame(step);
    })(segStart);
  }
})();

// RangeSliderValueTooltipFollow — vanilla 판(run/334/demo/01-range-slider-value-tooltip-follow/index.html)
// 의 React 포트. (value - min) / (max - min) * 100 을 --pct 로 내려 풍선의 left 를
// calc(var(--pct) * 1%) 로 계산한다. CSS @keyframes 는 없다 — 자동 시연은 목표값 사이를
// 지수 감쇠로 부드럽게 접근하는 requestAnimationFrame 루프다.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./RangeSliderValueTooltipFollow.module.scss";

export type RangeSliderValueTooltipFollowProps = {
  duration?: string;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };

const HINT = "퍼센트를 계산해 CSS 변수에 담으면 풍선이 썸을 그대로 따라다닙니다";
const ARIA_SCENE = "값이 바뀔 때마다 풍선 라벨이 썸을 따라 이동하는 장면";
const ARIA_RANGE = "값 슬라이더";
const UNIT = "calc 로 구한 풍선 위치";

const MIN = 0, MAX = 100;
const TAU = 0.11; // 지수 감쇠 시간상수(초) — 값 접근 속도
const TARGETS = [50, 18, 82, 35, 92, 50]; // 마지막 = 처음, 루프가 이어 붙는다
const SEG_MS = 400;

export default function RangeSliderValueTooltipFollow({
  duration = "220ms",
  easing = "ease-out",
  color = "#0a4fe8"
}: RangeSliderValueTooltipFollowProps) {
  const wrapRef = useRef<HTMLDivElement | null>(null);
  const valRef = useRef<HTMLElement | null>(null);
  const calcRef = useRef<HTMLElement | null>(null);
  const rangeRef = useRef<HTMLInputElement | null>(null);
  const rafRef = useRef(0) as Box<number>;

  useEffect(() => {
    const wrap = wrapRef.current;
    const valEl = valRef.current;
    const calcEl = calcRef.current;
    const range = rangeRef.current;
    if (!wrap || !valEl || !calcEl || !range) return;

    let current = 50;

    function paint(v: number): void {
      const pct = ((v - MIN) / (MAX - MIN)) * 100;
      wrap!.style.setProperty("--pct", pct.toFixed(2));
      valEl!.textContent = String(Math.round(v));
      calcEl!.textContent = pct.toFixed(1) + "%";
      range!.setAttribute("aria-valuenow", String(Math.round(v)));
    }
    paint(current);

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    let playing = true;
    function stop(): void {
      playing = false;
    }
    function onUserInput(): void {
      stop();
      current = Number(range!.value);
      paint(current);
    }
    range.addEventListener("input", onUserInput);
    range.addEventListener("pointerdown", stop);
    range.addEventListener("keydown", stop);

    if (reduce) {
      current = TARGETS[0];
      range.value = String(current);
      paint(current);
    } else {
      let seg = 0;
      let segStart = performance.now();
      let segFrom = current;
      const step = (now: number): void => {
        if (playing) {
          let elapsed = now - segStart;
          if (elapsed >= SEG_MS) {
            seg = (seg + 1) % TARGETS.length;
            segStart = now;
            segFrom = current;
            elapsed = 0;
          }
          const target = TARGETS[seg];
          const t = elapsed / 1000;
          const k = 1 - Math.exp(-t / TAU);
          current = segFrom + (target - segFrom) * k;
          range.value = String(Math.round(current));
          paint(current);
        }
        rafRef.current = requestAnimationFrame(step);
      };
      rafRef.current = requestAnimationFrame(step);
    }

    return () => {
      cancelAnimationFrame(rafRef.current);
      range.removeEventListener("input", onUserInput);
      range.removeEventListener("pointerdown", stop);
      range.removeEventListener("keydown", stop);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <main className={styles.stage}>
      <div
        className={styles.vt}
        style={{ "--duration": duration, "--easing": easing, "--color": color } as Vars}
      >
        <p className={styles.hint}>{HINT}</p>
        <div className={styles.wrap} ref={wrapRef} role="img" aria-label={ARIA_SCENE}>
          <div className={styles.bubble}>
            <span ref={valRef}>50</span>
          </div>
          <input
            className={styles.range}
            ref={rangeRef}
            type="range"
            min={0}
            max={100}
            step={1}
            defaultValue={50}
            aria-label={ARIA_RANGE}
          />
        </div>
        <p className={styles.meta}>
          <b className={styles.tag} ref={calcRef}>50.0%</b>
          <span className={styles.unit}>{UNIT}</span>
        </p>
      </div>
    </main>
  );
}