04Width-inertia value toggle — while an expand value grows, a width value overshoots its target and returns

<main class="stage">
<div class="we is-demo" id="we">
  <p class="we__line">This is a line of body text. The button below stretches with restraint so the line stays readable.</p>
  <button type="button" class="we__btn" id="weBtn" aria-expanded="false">
    <span class="we__base">More</span>
    <span class="we__more" id="weMore">View full value</span>
  </button>
</div>
</main>

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

// 계산 겹: 너비가 CSS transition 이 아니라 index.html 의 스프링 감쇠 계산(K·D)으로 매 프레임
// 결정된다 — 목표를 넘었다가 되돌아오는 관성. more 글자는 진행률 p 로만 얹힌다.
@include stage($stage-paper);

.we {
  width: 100%;
  max-width: 340px;
  display: grid;
  gap: $sp-4;
  justify-items: start;
}

.we__line {
  margin: 0;
  color: $subject-ink;
  font-size: 13px;
  line-height: 1.7;
}

.we__btn {
  position: relative;
  display: inline-flex;
  align-items: center;
  justify-content: flex-start;
  height: 44px;
  padding: 0 $sp-4;
  box-sizing: border-box;
  border: 0;
  border-radius: $r-pill;
  background: $subject-blue-deep;
  color: $subject-cream;
  font-weight: 600;
  font-size: 14px;
  cursor: pointer;
  overflow: hidden;
  white-space: nowrap;
  will-change: width;
}

.we__more {
  margin-left: $sp-2;
  opacity: 0;
  white-space: nowrap;
  font-weight: 600;
}

@media (max-width: 340px) {
  .we { max-width: 280px; }
  .we__btn { height: 40px; font-size: 13px; }
}

@media (prefers-reduced-motion: reduce) {
  .we__more { transition: none; }
}

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);
}

.we {
  width: 100%;
  max-width: 340px;
  display: grid;
  gap: 16px;
  justify-items: start;
}

.we__line {
  margin: 0;
  color: #17141a;
  font-size: 13px;
  line-height: 1.7;
}

.we__btn {
  position: relative;
  display: inline-flex;
  align-items: center;
  justify-content: flex-start;
  height: 44px;
  padding: 0 16px;
  box-sizing: border-box;
  border: 0;
  border-radius: 999px;
  background: #0a4fe8;
  color: #fff7e6;
  font-weight: 600;
  font-size: 14px;
  cursor: pointer;
  overflow: hidden;
  white-space: nowrap;
  will-change: width;
}

.we__more {
  margin-left: 8px;
  opacity: 0;
  white-space: nowrap;
  font-weight: 600;
}

@media (max-width: 340px) {
  .we {
    max-width: 280px;
  }
  .we__btn {
    height: 40px;
    font-size: 13px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .we__more {
    transition: none;
  }
}

(function () {
  var root = document.getElementById('we');
  var btn = document.getElementById('weBtn');
  var more = document.getElementById('weMore');
  var REST_W = 104; // 접힘 너비(px) — getBoundingClientRect 로 실측해 갱신한다
  var K = 170, D = 12; // 스프링 상수·감쇠 — 목표를 넘었다가 되돌아오는 관성 값

  // 계산 겹: 목표 폭을 getBoundingClientRect 로 실측한다(짐작 px 아님).
  // 늘어난 상태의 텍스트를 잠깐 펼쳐 재고 다시 접는다 — 글꼴마다 값이 다르다.
  function measure() {
    var prev = more.style.width;
    more.style.width = 'auto';
    var w = more.getBoundingClientRect().width;
    more.style.width = prev || '0px';
    return Math.ceil(w);
  }
  var openW = measure() + REST_W + 24; // 24 = 기준 너비에 더할 여유

  var open = false;
  var cur = REST_W, vel = 0, target = REST_W;
  var raf = null;

  // 스프링 감쇠 계산: 매 프레임 a = -K*(cur-target) - D*vel — 오버슈트 후 안착
  function tick() {
    var a = -K * (cur - target) * 0.001 - D * vel * 0.01;
    vel += a;
    cur += vel;
    if (Math.abs(cur - target) < 0.4 && Math.abs(vel) < 0.05) { cur = target; vel = 0; }
    btn.style.width = cur.toFixed(1) + 'px';
    var p = Math.max(0, Math.min(1, (cur - REST_W) / (openW - REST_W)));
    more.style.opacity = String(p);
    raf = requestAnimationFrame(tick);
  }

  function toggle() {
    open = !open;
    target = open ? openW : REST_W;
    btn.setAttribute('aria-expanded', String(open));
  }

  btn.addEventListener('click', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    toggle();
  });

  raf = requestAnimationFrame(tick);

  var autoTimer = null;
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (!reduce) { autoTimer = setInterval(toggle, 1300); }
  else { open = true; target = openW; cur = openW; btn.style.width = openW + 'px'; more.style.opacity = '1'; }
})();

import { useEffect, useRef } from "react";
import styles from "./WidthExpandInline.module.scss";

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

const REST_W = 104; // 접힘 너비(px) — getBoundingClientRect 로 실측해 갱신한다
const K = 170, D = 12; // 스프링 상수·감쇠 — 목표를 넘었다가 되돌아오는 관성 값

export default function WidthExpandInline() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement | null>;

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const btn = root.querySelector(`.${styles.btn}`) as HTMLButtonElement | null;
    const more = root.querySelector(`.${styles.more}`) as HTMLElement | null;
    if (!btn || !more) return;

    // 계산 겹: 목표 폭을 getBoundingClientRect 로 실측한다(짐작 px 아님).
    function measure() {
      const prev = more!.style.width;
      more!.style.width = "auto";
      const w = more!.getBoundingClientRect().width;
      more!.style.width = prev || "0px";
      return Math.ceil(w);
    }
    const openW = measure() + REST_W + 24; // 24 = 기준 너비에 더할 여유

    let open = false;
    let cur = REST_W, vel = 0, target = REST_W;
    let raf: number | null = null;

    function tick() {
      const a = -K * (cur - target) * 0.001 - D * vel * 0.01;
      vel += a;
      cur += vel;
      if (Math.abs(cur - target) < 0.4 && Math.abs(vel) < 0.05) { cur = target; vel = 0; }
      btn!.style.width = cur.toFixed(1) + "px";
      const p = Math.max(0, Math.min(1, (cur - REST_W) / (openW - REST_W)));
      more!.style.opacity = String(p);
      raf = requestAnimationFrame(tick);
    }

    function toggle() {
      open = !open;
      target = open ? openW : REST_W;
      btn!.setAttribute("aria-expanded", String(open));
    }

    function onClick() {
      root!.classList.remove(styles["is-demo"]);
      stopAutoplay();
      toggle();
    }
    btn.addEventListener("click", onClick);

    raf = requestAnimationFrame(tick);

    let autoTimer: number | null = null;
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) { autoTimer = window.setInterval(toggle, 1300); }
    else {
      open = true; target = openW; cur = openW;
      btn!.style.width = openW + "px"; more!.style.opacity = "1";
    }

    return () => {
      stopAutoplay();
      if (raf !== null) cancelAnimationFrame(raf);
      btn.removeEventListener("click", onClick);
    };
  }, []);

  return (
    <div ref={rootRef} className={`${styles.stage} ${styles.we} ${styles["is-demo"]}`}>
      <p className={styles.line}>본문 한 줄이 흐르는 자리에 있습니다. 아래 단추가 그 줄을 흐리지 않게 절제되게 늘어납니다.</p>
      <button type="button" className={styles.btn} aria-expanded="false">
        <span className={styles.base}>더 보기</span>
        <span className={styles.more}>전문 값 보기</span>
      </button>
    </div>
  );
}