02ばねで滑るアクティブなピル

<main class="stage">
<div class="pg is-demo" id="pgRoot">
  <nav class="pg__nav" id="pgNav" aria-label="ページ移動">
    <span class="pg__pill" id="pgPill" aria-hidden="true"></span>
  </nav>
  <div class="pg__read">
    <span class="pg__k">オーバーシュート</span><b id="pgOver">0</b><span class="pg__u">%</span>
  </div>
  <p class="pg__sr" id="pgSr" aria-live="polite"></p>
</div>
</main>

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

// 핵심: 알약 위치는 JS 가 스프링으로 적분한 값이다 (PILL_STIFFNESS · PILL_DAMPING).
// 속도에 비례해 알약이 늘어나고(scale) 도착점을 넘었다 돌아온다.

$slot-w: 36px;
$slot-gap: 4px;

@include stage($stage-ink);

.pg { display: grid; gap: $sp-4; justify-items: center; color: $stage-paper; }

.pg__nav {
  position: relative;
  display: flex;
  gap: $slot-gap;
  padding: $sp-1;
  border-radius: $r-pill;
  background: rgba($stage-paper, .1);
}

.pg__pill {
  position: absolute;
  left: $sp-1;
  top: $sp-1;
  width: $slot-w;
  height: $slot-w;
  border-radius: $r-pill;
  background: $subject-lilac;
  transform-origin: 50% 50%;
  will-change: transform;
}

.pg__b {
  position: relative;
  box-sizing: border-box;
  width: $slot-w;
  height: $slot-w;
  padding: 0;
  border: 0;
  border-radius: $r-pill;
  background: transparent;
  color: $stage-paper;
  font-family: inherit;
  font-size: 13px;
  line-height: 16px;
  font-weight: 600;
  font-variant-numeric: tabular-nums;
  cursor: pointer;
}

.pg__b.is-under { color: $subject-ink; }
.pg__b:focus-visible { outline: 2px solid $subject-sky; outline-offset: 2px; }

.pg__read { display: flex; align-items: baseline; gap: $sp-1; font-size: 12px; line-height: 16px; }
.pg__k { color: rgba($stage-paper, .82); margin-right: $sp-1; }
.pg__u { color: rgba($stage-paper, .82); }
.pg__read b { color: $subject-pink; font-variant-numeric: tabular-nums; min-width: 24px; text-align: right; }

.pg__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: 0;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
}

@media (max-width: 340px) {
  .pg__b { font-size: 12px; }
}

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

.pg {
  display: grid;
  gap: 16px;
  justify-items: center;
  color: #ffffff;
}

.pg__nav {
  position: relative;
  display: flex;
  gap: 4px;
  padding: 4px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.1);
}

.pg__pill {
  position: absolute;
  left: 4px;
  top: 4px;
  width: 36px;
  height: 36px;
  border-radius: 999px;
  background: #b9a0ff;
  transform-origin: 50% 50%;
  will-change: transform;
}

.pg__b {
  position: relative;
  box-sizing: border-box;
  width: 36px;
  height: 36px;
  padding: 0;
  border: 0;
  border-radius: 999px;
  background: transparent;
  color: #ffffff;
  font-family: inherit;
  font-size: 13px;
  line-height: 16px;
  font-weight: 600;
  font-variant-numeric: tabular-nums;
  cursor: pointer;
}

.pg__b.is-under {
  color: #17141a;
}

.pg__b:focus-visible {
  outline: 2px solid #6ec8ff;
  outline-offset: 2px;
}

.pg__read {
  display: flex;
  align-items: baseline;
  gap: 4px;
  font-size: 12px;
  line-height: 16px;
}

.pg__k {
  color: rgba(255, 255, 255, 0.82);
  margin-right: 4px;
}

.pg__u {
  color: rgba(255, 255, 255, 0.82);
}

.pg__read b {
  color: #ff9ebb;
  font-variant-numeric: tabular-nums;
  min-width: 24px;
  text-align: right;
}

.pg__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: 0;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
}

@media (max-width: 340px) {
  .pg__b {
    font-size: 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .pg__pill {
    transition: none;
  }
}

(function () {
  // 기법: 알약의 x 위치를 CSS 전환이 아니라 스프링 방정식으로 직접 적분한다.
  // 가속도 = -강성 x (위치 - 목표) - 감쇠 x 速度. 속도가 클수록 알약이 진행 방향으로 늘어난다.
  var PAGE_COUNT = 7;
  var SLOT_W = 36;
  var SLOT_GAP = 4;
  var PILL_STIFFNESS = 320;
  var PILL_DAMPING = 17;
  var STRETCH_GAIN = 0.0011;
  var STRETCH_MAX = 0.7;
  var SUBSTEP = 1 / 240;
  var STEP_MS = 400;
  var START_MS = 150;
  var DEMO_PAGES = [1, 7, 3, 6, 1, 4, 7];

  var root = document.getElementById('pgRoot');
  var nav = document.getElementById('pgNav');
  var pill = document.getElementById('pgPill');
  var overEl = document.getElementById('pgOver');
  var sr = document.getElementById('pgSr');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var btns = [];
  for (var i = 0; i < PAGE_COUNT; i += 1) {
    var b = document.createElement('button');
    b.type = 'button';
    b.className = 'pg__b';
    b.textContent = String(i + 1);
    nav.appendChild(b);
    btns.push(b);
  }

  function slotX(page) { return (page - 1) * (SLOT_W + SLOT_GAP); }

  var cur = 1;
  var x = 0;
  var v = 0;
  var fromX = 0;
  var peak = 0;
  var lastTs = null;
  var t0 = null;
  var lastStep = -1;

  function setCur(n, byUser) {
    var next = Math.max(1, Math.min(PAGE_COUNT, n));
    if (byUser) root.classList.remove('is-demo');
    if (next === cur) return;
    cur = next;
    fromX = x;
    peak = 0;
    for (var s = 0; s < PAGE_COUNT; s += 1) {
      if (s + 1 === cur) btns[s].setAttribute('aria-current', 'page'); else btns[s].removeAttribute('aria-current');
    }
    if (reduce) { x = slotX(cur); v = 0; paint(); }
    if (byUser) sr.textContent = '現在 ' + cur + ' / ' + PAGE_COUNT;
  }

  function paint() {
    var sx = 1 + Math.min(STRETCH_MAX, Math.abs(v) * STRETCH_GAIN);
    var sy = 1 / Math.sqrt(sx);
    pill.style.transform = 'translateX(' + x.toFixed(2) + 'px) scale(' + sx.toFixed(3) + ',' + sy.toFixed(3) + ')';
    var near = Math.max(0, Math.min(PAGE_COUNT - 1, Math.round(x / (SLOT_W + SLOT_GAP))));
    for (var s = 0; s < PAGE_COUNT; s += 1) btns[s].className = 'pg__b' + (s === near ? ' is-under' : '');
  }

  nav.addEventListener('click', function (e) {
    var idx = btns.indexOf(e.target);
    if (idx >= 0) setCur(idx + 1, true);
  });
  nav.addEventListener('keydown', function (e) {
    var to = null;
    if (e.key === 'ArrowRight') to = cur + 1;
    else if (e.key === 'ArrowLeft') to = cur - 1;
    else if (e.key === 'Home') to = 1;
    else if (e.key === 'End') to = PAGE_COUNT;
    if (to === null) return;
    e.preventDefault();
    setCur(to, true);
    btns[Math.max(1, Math.min(PAGE_COUNT, to)) - 1].focus({ preventScroll: true });
  });

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    if (root.classList.contains('is-demo')) {
      var step = now < START_MS ? 0 : Math.floor((now - START_MS) / STEP_MS) % DEMO_PAGES.length;
      if (step !== lastStep) { lastStep = step; setCur(DEMO_PAGES[step], false); }
    }
    var target = slotX(cur);
    var left = dt;
    while (left > 0) {
      var h = Math.min(SUBSTEP, left);
      v += (-PILL_STIFFNESS * (x - target) - PILL_DAMPING * v) * h;
      x += v * h;
      left -= h;
    }
    var span = target - fromX;
    if (span !== 0) {
      var over = ((x - target) * Math.sign(span)) / Math.abs(span);
      if (over > peak) peak = over;
      overEl.textContent = String(Math.round(peak * 100));
    }
    paint();
    requestAnimationFrame(tick);
  }

  btns[0].setAttribute('aria-current', 'page');
  if (reduce) {
    root.classList.remove('is-demo');
    cur = 4;
    x = slotX(cur);
    for (var q = 0; q < PAGE_COUNT; q += 1) {
      if (q + 1 === cur) btns[q].setAttribute('aria-current', 'page'); else btns[q].removeAttribute('aria-current');
    }
    paint();
  } else {
    paint();
    requestAnimationFrame(tick);
  }
})();

// SpringPillPager — vanilla 판(02-pager-active-pill-spring-slide-overshoot) 의 React 포트.
// 알약의 x 위치를 CSS 전환이 아니라 스프링 방정식(강성·감쇠)으로 직접 적분하고, 속도에 비례해 늘어나게 한다.
import { useEffect, useRef } from "react";
import styles from "./SpringPillPager.module.scss";

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

const PAGE_COUNT = 7;
const SLOT_W = 36;
const SLOT_GAP = 4;
const PILL_STIFFNESS = 320;
const PILL_DAMPING = 17;
const STRETCH_GAIN = 0.0011;
const STRETCH_MAX = 0.7;
const SUBSTEP = 1 / 240;
const STEP_MS = 400;
const START_MS = 150;
const DEMO_PAGES = [1, 7, 3, 6, 1, 4, 7];

export default function SpringPillPager() {
  const navRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;
  const pillRef = useRef<HTMLSpanElement | null>(null) as Box<HTMLSpanElement | null>;
  const overRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;
  const srRef = useRef<HTMLParagraphElement | null>(null) as Box<HTMLParagraphElement | null>;
  const btnRefs = useRef<Array<HTMLButtonElement | null>>([]) as Box<Array<HTMLButtonElement | null>>;

  useEffect(() => {
    const nav = navRef.current;
    const pill = pillRef.current;
    const overEl = overRef.current;
    const sr = srRef.current;
    if (!nav || !pill || !overEl || !sr) return;
    const btns = btnRefs.current;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const slotX = (page: number): number => (page - 1) * (SLOT_W + SLOT_GAP);

    let demo = true;
    let cur = 1;
    let x = 0;
    let v = 0;
    let fromX = 0;
    let peak = 0;
    let lastTs: number | null = null;
    let t0: number | null = null;
    let lastStep = -1;
    let raf = 0;

    function mark(): void {
      for (let s = 0; s < PAGE_COUNT; s += 1) {
        const el = btns[s];
        if (!el) continue;
        if (s + 1 === cur) el.setAttribute("aria-current", "page"); else el.removeAttribute("aria-current");
      }
    }

    function paint(): void {
      const sx = 1 + Math.min(STRETCH_MAX, Math.abs(v) * STRETCH_GAIN);
      const sy = 1 / Math.sqrt(sx);
      pill!.style.transform = "translateX(" + x.toFixed(2) + "px) scale(" + sx.toFixed(3) + "," + sy.toFixed(3) + ")";
      const near = Math.max(0, Math.min(PAGE_COUNT - 1, Math.round(x / (SLOT_W + SLOT_GAP))));
      for (let s = 0; s < PAGE_COUNT; s += 1) {
        const el = btns[s];
        if (el) el.className = [styles.pg__b, s === near ? styles["is-under"] : ""].join(" ");
      }
    }

    function setCur(n: number, byUser: boolean): void {
      const next = Math.max(1, Math.min(PAGE_COUNT, n));
      if (byUser) demo = false;
      if (next === cur) return;
      cur = next;
      fromX = x;
      peak = 0;
      mark();
      if (reduce) { x = slotX(cur); v = 0; paint(); }
      if (byUser) sr!.textContent = "현재 " + cur + " / " + PAGE_COUNT;
    }

    const onClick = (e: MouseEvent): void => {
      const idx = btns.indexOf(e.target as HTMLButtonElement);
      if (idx >= 0) setCur(idx + 1, true);
    };
    const onKey = (e: KeyboardEvent): void => {
      let to: number | null = null;
      if (e.key === "ArrowRight") to = cur + 1;
      else if (e.key === "ArrowLeft") to = cur - 1;
      else if (e.key === "Home") to = 1;
      else if (e.key === "End") to = PAGE_COUNT;
      if (to === null) return;
      e.preventDefault();
      setCur(to, true);
      const target = btns[Math.max(1, Math.min(PAGE_COUNT, to)) - 1];
      if (target) target.focus({ preventScroll: true });
    };
    nav.addEventListener("click", onClick);
    nav.addEventListener("keydown", onKey);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      if (demo) {
        const step = now < START_MS ? 0 : Math.floor((now - START_MS) / STEP_MS) % DEMO_PAGES.length;
        if (step !== lastStep) { lastStep = step; setCur(DEMO_PAGES[step], false); }
      }
      const target = slotX(cur);
      let left = dt;
      while (left > 0) {
        const h = Math.min(SUBSTEP, left);
        v += (-PILL_STIFFNESS * (x - target) - PILL_DAMPING * v) * h;
        x += v * h;
        left -= h;
      }
      const span = target - fromX;
      if (span !== 0) {
        const over = ((x - target) * Math.sign(span)) / Math.abs(span);
        if (over > peak) peak = over;
        overEl!.textContent = String(Math.round(peak * 100));
      }
      paint();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) {
      demo = false;
      cur = 4;
      x = slotX(cur);
      mark();
      paint();
    } else {
      mark();
      paint();
      raf = requestAnimationFrame(tick);
    }
    return () => {
      cancelAnimationFrame(raf);
      nav.removeEventListener("click", onClick);
      nav.removeEventListener("keydown", onKey);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.pg}>
        <nav className={styles.pg__nav} ref={navRef} aria-label="페이지 이동">
          <span className={styles.pg__pill} ref={pillRef} aria-hidden="true"></span>
          {Array.from({ length: PAGE_COUNT }, (_, i) => (
            <button type="button" className={styles.pg__b} key={i} ref={(el: HTMLButtonElement | null) => { btnRefs.current[i] = el; }}>{i + 1}</button>
          ))}
        </nav>
        <div className={styles.pg__read}>
          <span className={styles.pg__k}>오버슛</span><b ref={overRef}>0</b><span className={styles.pg__u}>%</span>
        </div>
        <p className={styles.pg__sr} ref={srRef} aria-live="polite"></p>
      </div>
    </main>
  );
}