03An Input Whose Underline Chases the Caret with Inertia

<main class="stage">
  <div class="fl is-demo" id="fl" data-demo="1">
    <div class="fl__field">
      <div class="fl__box" data-r="box">
        <input class="fl__input" id="f1" data-r="input" type="text" autocomplete="email" inputmode="email">
        <label class="fl__label" for="f1" data-r="label">Email address</label>
        <span class="fl__mirror" data-r="mirror" aria-hidden="true"></span>
        <i class="fl__line" aria-hidden="true"></i>
        <i class="fl__trail" data-r="trail" aria-hidden="true"></i>
        <i class="fl__dot" data-r="dot" aria-hidden="true"></i>
      </div>
    </div>
    <div class="fl__field">
      <div class="fl__box" data-r="box">
        <input class="fl__input" id="f2" data-r="input" type="text" autocomplete="name">
        <label class="fl__label" for="f2" data-r="label">Name</label>
        <span class="fl__mirror" data-r="mirror" aria-hidden="true"></span>
        <i class="fl__line" aria-hidden="true"></i>
        <i class="fl__trail" data-r="trail" aria-hidden="true"></i>
        <i class="fl__dot" data-r="dot" aria-hidden="true"></i>
      </div>
    </div>
  </div>
</main>

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

// 핵심: 캐럿 앞 글자의 폭을 재서 밑줄 빛 점이 그 x 로 관성 있게 따라간다. 라벨은 진짜 <label for> 이고 칸은 진짜 입력 요소다. 움직임 값은 JS 가 계산하고 CSS 는 모양만 맡는다.

@include stage($stage-paper);

$panel: mix($subject-lilac, $stage-paper, 16%);
$box-bg: $stage-paper;
$fg: $subject-ink;
$mute: mix($subject-ink, $stage-paper, 62%);
$line: mix($subject-ink, $stage-paper, 30%);
$accent: $subject-blue-deep;
$err: $error;
$menu-bg: $stage-paper;
$box-h: 52px;

.fl {
  position: relative;
  box-sizing: border-box;
  width: min(100%, 440px);
  padding: $sp-5;
  display: flex;
  flex-direction: column;
  gap: $sp-4;
  border-radius: $r-card;
  background: $panel;
  color: $fg;
  font-size: 16px;
  line-height: 24px;
  font-weight: 500;
}
.fl__field { position: relative; }
.fl__box {
  --ln: #{$line};
  position: relative;
  z-index: 1;
  height: $box-h;
  border-radius: $r-sm;
  background: $box-bg;
  box-shadow: inset 0 0 0 1.5px var(--ln), 0 0 0 calc(var(--p, 0) * 3px) rgba($accent, .3);
}
.fl__box[data-up] { --ln: #{$accent}; }
.fl__box::after {
  content: "";
  position: absolute;
  inset: 0;
  z-index: 0;
  border-radius: inherit;
  background: $accent;
  opacity: calc(var(--p, 0) * .12);
  pointer-events: none;
}
.fl__input {
  position: relative;
  z-index: 1;
  box-sizing: border-box;
  display: block;
  width: 100%;
  height: 100%;
  margin: 0;
  padding: $sp-6 $sp-3 $sp-1;
  border: 0;
  border-radius: inherit;
  background: transparent;
  color: $fg;
  font-family: inherit;
  font-size: 16px;
  line-height: 24px;
  font-weight: 500;
  outline: none;
}
.fl__input:focus-visible { outline: 2px solid $accent; outline-offset: 2px; }
.fl__label {
  position: absolute;
  left: $sp-3;
  top: 0;
  z-index: 2;
  line-height: $box-h;
  font-size: 16px;
  font-weight: 500;
  color: $mute;
  white-space: nowrap;
  transform-origin: 0 50%;
  pointer-events: none;
  will-change: transform;
}
.fl__box[data-up] .fl__label { color: $accent; }
.fl__box { box-shadow: none; border-radius: $r-sm $r-sm 0 0; overflow: hidden; }
.fl__line { position: absolute; left: 0; right: 0; bottom: 0; height: 4px; background: $line; z-index: 2; }
.fl__trail { position: absolute; left: 0; bottom: 0; width: 100%; height: 4px; background: $accent; z-index: 3; opacity: 0; transform: scaleX(0); transform-origin: 0 50%; will-change: transform; }
.fl__dot {
  position: absolute;
  left: 0;
  bottom: 0;
  width: 44px;
  height: 4px;
  z-index: 4;
  border-radius: $r-pill;
  background: radial-gradient(closest-side, $accent, rgba($accent, 0));
  box-shadow: 0 0 14px 4px rgba($accent, .6);
  opacity: 0;
  will-change: transform;
}
.fl__mirror {
  position: absolute;
  left: 0;
  top: 0;
  visibility: hidden;
  white-space: pre;
  font-family: inherit;
  font-size: 16px;
  line-height: 24px;
  font-weight: 500;
  pointer-events: none;
}
@media (max-height: 250px) {
  .fl__field:nth-child(n + 3) { display: none; }
}
@media (prefers-reduced-motion: reduce) {
  .fl__label, .fl__box { will-change: auto; }
}

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);
}

.fl {
  position: relative;
  box-sizing: border-box;
  width: min(100%, 440px);
  padding: 20px;
  display: flex;
  flex-direction: column;
  gap: 16px;
  border-radius: 16px;
  background: rgb(95.6078431373%, 94.0392156863%, 100%);
  color: #17141a;
  font-size: 16px;
  line-height: 24px;
  font-weight: 500;
}

.fl__field {
  position: relative;
}

.fl__box {
  --ln: rgb(72.7058823529%, 72.3529411765%, 73.0588235294%);
  position: relative;
  z-index: 1;
  height: 52px;
  border-radius: 8px;
  background: #ffffff;
  box-shadow: inset 0 0 0 1.5px var(--ln), 0 0 0 calc(var(--p, 0) * 3px) rgba(10, 79, 232, 0.3);
}

.fl__box[data-up] {
  --ln: #0a4fe8;
}

.fl__box::after {
  content: "";
  position: absolute;
  inset: 0;
  z-index: 0;
  border-radius: inherit;
  background: #0a4fe8;
  opacity: calc(var(--p, 0) * 0.12);
  pointer-events: none;
}

.fl__input {
  position: relative;
  z-index: 1;
  box-sizing: border-box;
  display: block;
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 24px 12px 4px;
  border: 0;
  border-radius: inherit;
  background: transparent;
  color: #17141a;
  font-family: inherit;
  font-size: 16px;
  line-height: 24px;
  font-weight: 500;
  outline: none;
}

.fl__input:focus-visible {
  outline: 2px solid #0a4fe8;
  outline-offset: 2px;
}

.fl__label {
  position: absolute;
  left: 12px;
  top: 0;
  z-index: 2;
  line-height: 52px;
  font-size: 16px;
  font-weight: 500;
  color: rgb(43.5921568627%, 42.862745098%, 44.3215686275%);
  white-space: nowrap;
  transform-origin: 0 50%;
  pointer-events: none;
  will-change: transform;
}

.fl__box[data-up] .fl__label {
  color: #0a4fe8;
}

.fl__box {
  box-shadow: none;
  border-radius: 8px 8px 0 0;
  overflow: hidden;
}

.fl__line {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 4px;
  background: rgb(72.7058823529%, 72.3529411765%, 73.0588235294%);
  z-index: 2;
}

.fl__trail {
  position: absolute;
  left: 0;
  bottom: 0;
  width: 100%;
  height: 4px;
  background: #0a4fe8;
  z-index: 3;
  opacity: 0;
  transform: scaleX(0);
  transform-origin: 0 50%;
  will-change: transform;
}

.fl__dot {
  position: absolute;
  left: 0;
  bottom: 0;
  width: 44px;
  height: 4px;
  z-index: 4;
  border-radius: 999px;
  background: radial-gradient(closest-side, #0a4fe8, rgba(10, 79, 232, 0));
  box-shadow: 0 0 14px 4px rgba(10, 79, 232, 0.6);
  opacity: 0;
  will-change: transform;
}

.fl__mirror {
  position: absolute;
  left: 0;
  top: 0;
  visibility: hidden;
  white-space: pre;
  font-family: inherit;
  font-size: 16px;
  line-height: 24px;
  font-weight: 500;
  pointer-events: none;
}

@media (max-height: 250px) {
  .fl__field:nth-child(n+3) {
    display: none;
  }
}
@media (prefers-reduced-motion: reduce) {
  .fl__label, .fl__box {
    will-change: auto;
  }
}

(function () {
    const root = document.getElementById("fl");
    const CLS = {};
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let raf = 0;
    let last = -1;
    const offs = [];
    function q(sel) {
        return root.querySelector(sel);
    }
    function qa(sel) {
        const list = Array.prototype.slice.call(root.querySelectorAll(sel));
        return list;
    }
    function clamp(v, lo, hi) {
        return Math.max(lo, Math.min(hi, v));
    }
    function on(el, type, fn) {
        el.addEventListener(type, fn);
        offs.push(function () {
            el.removeEventListener(type, fn);
        });
    }
    // 좌표 해시: 같은 입력이면 같은 값 0~1. 난수 함수를 쓰지 않고 사인의 소수부만 쓴다.
    function hash(x) {
        const s = Math.sin(x * 127.1 + 311.7) * 43758.5453;
        return s - Math.floor(s);
    }
    // 지수 접근: 프레임당 비율 per 만큼 목표에 다가간다. f 는 60fps 기준 프레임 수(dt * 60)라 프레임률이 달라도 같은 속도다.
    function approach(x, target, per, f) {
        return x + (target - x) * (1 - Math.pow(1 - per, f));
    }
    // 스프링 한 칸: 가속도 = -k (x - 목표) - c v. 반암시적 오일러를 1/120초 이하 부분 스텝으로 쪼갠다.
    function springStep(s, target, k, c, dt) {
        let left = dt;
        while (left > 0) {
            const h = Math.min(left, 1 / 120);
            s.v += (-k * (s.x - target) - c * s.v) * h;
            s.x += s.v * h;
            left -= h;
        }
    }
    // 값이 있으면 라벨은 올라가 있어야 한다. 브라우저 자동완성이 값을 채우면 input 이벤트가 없을 수 있어 :autofill 도 함께 본다.
    function isFilled(el) {
        if (el.value !== "") return true;
        try {
            return el.matches(":-webkit-autofill");
        } catch (err) {
            return false;
        }
    }
    let leaveHook = null;
    function stopDemo() {
        if (!demo) return;
        demo = false;
        root.removeAttribute("data-demo");
        root.classList.remove("is-demo");
        resetFields();
    }
    function frame(ts) {
        const dt = last < 0 ? 0.016 : Math.min(0.05, (ts - last) / 1000);
        last = ts;
        if (demo) script(ts);
        step(dt, ts);
        render();
        raf = requestAnimationFrame(frame);
    }
    const ro = new ResizeObserver(function () {
        relayout();
    });
    ro.observe(root);
    if (document.fonts) {
        document.fonts.ready.then(function () {
            relayout();
        });
    }

    // 시연 계획: 칸마다 포커스 구간(a~b), 타이핑 시작(s)·글자당 ms(rate)·글. 2초 주기로 되풀이하고 주기 첫머리엔 모두 비운다.
    function runPlan(ts) {
        const t = ts % 2000;
        let changed = false;
        for (let i = 0; i < PLAN.length; i += 1) {
            const p = PLAN[i];
            const f = fields[i];
            const isOn = t >= p.a && t < p.b;
            const n = t < p.s ? 0 : Math.min(p.text.length, Math.floor((t - p.s) / p.rate) + 1);
            const v = p.text.slice(0, n);
            if (f.input.value !== v) {
                f.input.value = v;
                changed = true;
            }
            if (f.focus !== isOn) {
                f.focus = isOn;
                changed = true;
                if (!isOn && leaveHook) leaveHook(i);
            }
        }
        if (changed) retarget();
    }
    function resetFields() {
        for (let i = 0; i < fields.length; i += 1) {
            fields[i].input.value = "";
            fields[i].focus = false;
        }
        retarget();
    }
    function wire() {
        fields.forEach(function (f, i) {
            on(f.input, "focus", function () {
                stopDemo();
                f.focus = true;
                retarget();
            });
            on(f.input, "blur", function () {
                f.focus = false;
                if (leaveHook) leaveHook(i);
                retarget();
            });
            on(f.input, "input", function () {
                retarget();
            });
        });
    }

    // 기법: 캐럿 앞 글자의 폭을 거울 span 으로 재서 밑줄 빛 점이 그 x 로 지수 접근한다. 타이핑이 빠르면 점이 뒤처진다.
    const LERP = 0.18;
    const UNDERLINE_H = 4;
    const CARET_OFFSET = 1;
    const TRAIL_FADE = 0.45;
    const INPUT_PAD = 12;
    const RISE_PX = 15;
    const LABEL_SCALE = 0.74;
    const DOT_W = 44;
    const fields = qa("[data-r=box]").map(function (box) {
        const trail = box.querySelector("[data-r=trail]");
        const dot = box.querySelector("[data-r=dot]");
        trail.style.height = UNDERLINE_H + "px";
        dot.style.height = UNDERLINE_H + "px";
        dot.style.width = DOT_W + "px";
        return {
            box: box,
            input: box.querySelector("input"),
            label: box.querySelector("label"),
            mirror: box.querySelector("[data-r=mirror]"),
            trail: trail, dot: dot,
            focus: false, up: false, p: 0, x: INPUT_PAD, tx: INPUT_PAD, w: 400
        };
    });
    const PLAN = [
        { a: 200, b: 1150, s: 250, rate: 60, text: "[email protected]" },
        { a: 1150, b: 1980, s: 1250, rate: 90, text: "Mina Seo" }
    ];
    // 주의: type="email" 칸은 selectionStart 가 null 이고 setSelectionRange 가 예외를 던진다. 그래서 이메일 칸도 type="text" + inputmode="email" 로 둔다.
    // 캐럿 x = 안쪽 여백 + 캐럿 앞 글자 폭 + 보정. 글자 폭은 input 과 같은 글꼴의 거울 span 으로 잰다.
    function caretX(f) {
        const at = f.input.selectionStart;
        const n = at === null ? f.input.value.length : at;
        f.mirror.textContent = f.input.value.slice(0, n);
        return INPUT_PAD + f.mirror.getBoundingClientRect().width + CARET_OFFSET;
    }
    function retarget() {
        fields.forEach(function (f) {
            f.up = f.focus || isFilled(f.input);
            if (f.up) f.box.setAttribute("data-up", "");
            else f.box.removeAttribute("data-up");
            f.tx = caretX(f);
            if (reduce) {
                f.p = f.up ? 1 : 0;
                f.x = f.tx;
            }
        });
        if (reduce) render();
    }
    function relayout() {
        fields.forEach(function (f) {
            f.w = f.box.clientWidth;
        });
        retarget();
    }
    let jumped = false;
    function script(ts) {
        runPlan(ts);
        const t = ts % 2000;
        const f = fields[0];
        const want = t >= 1040 && t < 1150;
        if (want !== jumped) {
            jumped = want;
            const to = want ? 3 : f.input.value.length;
            f.input.setSelectionRange(to, to);
            retarget();
        }
    }
    function step(dt, ts) {
        const fr = dt * 60;
        fields.forEach(function (f) {
            f.p = approach(f.p, f.up ? 1 : 0, 0.22, fr);
            f.x = approach(f.x, f.tx, LERP, fr);
        });
    }
    function render() {
        fields.forEach(function (f) {
            const s = 1 - (1 - LABEL_SCALE) * f.p;
            f.label.style.transform = "translate3d(0," + (-RISE_PX * f.p).toFixed(2) + "px,0) scale(" + s.toFixed(3) + ")";
            f.box.style.setProperty("--p", f.p.toFixed(3));
            const lag = Math.abs(f.tx - f.x);
            const glow = clamp(TRAIL_FADE + lag / 80, 0, 1) * f.p;
            f.trail.style.transform = "scaleX(" + clamp(f.x / f.w, 0, 1).toFixed(4) + ")";
            f.trail.style.opacity = glow.toFixed(3);
            f.dot.style.transform = "translate3d(" + (f.x - DOT_W / 2).toFixed(2) + "px,0,0)";
            f.dot.style.opacity = f.p.toFixed(3);
        });
    }
    wire();

    relayout();
    if (reduce) {
        render();
    } else {
        raf = requestAnimationFrame(frame);
    }

})();

// InertiaCaretUnderline - vanilla 판(03-fl-inertia-caret-underline) 의 React 포트. 같은 계산을 useEffect 안에서 돌린다.
import { useEffect, useRef } from "react";
import styles from "./InertiaCaretUnderline.module.scss";

type Spring = { x: number; v: number };
type Plan = { a: number; b: number; s: number; rate: number; text: string };
type Base = { input: HTMLInputElement | HTMLTextAreaElement; focus: boolean; up: boolean };
type F = Base & { box: HTMLElement; label: HTMLElement; mirror: HTMLElement; trail: HTMLElement; dot: HTMLElement; p: number; x: number; tx: number; w: number };

export default function InertiaCaretUnderline() {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const rootEl = rootRef.current;
    if (!rootEl) return;
    const root: HTMLElement = rootEl;
    const CLS = {  };
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let raf: number = 0;
    let last: number = -1;
    const offs: Array<() => void> = [];
    function q(sel: string): HTMLElement {
        return root.querySelector(sel) as HTMLElement;
    }
    function qa(sel: string): HTMLElement[] {
        const list: HTMLElement[] = Array.prototype.slice.call(root.querySelectorAll(sel));
        return list;
    }
    function clamp(v: number, lo: number, hi: number): number {
        return Math.max(lo, Math.min(hi, v));
    }
    function on(el: EventTarget, type: string, fn: (e: any) => void): void {
        el.addEventListener(type, fn);
        offs.push(function (): void {
            el.removeEventListener(type, fn);
        });
    }
    // 좌표 해시: 같은 입력이면 같은 값 0~1. 난수 함수를 쓰지 않고 사인의 소수부만 쓴다.
    function hash(x: number): number {
        const s = Math.sin(x * 127.1 + 311.7) * 43758.5453;
        return s - Math.floor(s);
    }
    // 지수 접근: 프레임당 비율 per 만큼 목표에 다가간다. f 는 60fps 기준 프레임 수(dt * 60)라 프레임률이 달라도 같은 속도다.
    function approach(x: number, target: number, per: number, f: number): number {
        return x + (target - x) * (1 - Math.pow(1 - per, f));
    }
    // 스프링 한 칸: 가속도 = -k (x - 목표) - c v. 반암시적 오일러를 1/120초 이하 부분 스텝으로 쪼갠다.
    function springStep(s: Spring, target: number, k: number, c: number, dt: number): void {
        let left = dt;
        while (left > 0) {
            const h = Math.min(left, 1 / 120);
            s.v += (-k * (s.x - target) - c * s.v) * h;
            s.x += s.v * h;
            left -= h;
        }
    }
    // 값이 있으면 라벨은 올라가 있어야 한다. 브라우저 자동완성이 값을 채우면 input 이벤트가 없을 수 있어 :autofill 도 함께 본다.
    function isFilled(el: HTMLInputElement | HTMLTextAreaElement): boolean {
        if (el.value !== "") return true;
        try {
            return el.matches(":-webkit-autofill");
        } catch (err) {
            return false;
        }
    }
    let leaveHook: ((i: number) => void) | null = null;
    function stopDemo(): void {
        if (!demo) return;
        demo = false;
        root.removeAttribute("data-demo");
        root.classList.remove("is-demo");
        resetFields();
    }
    function frame(ts: number): void {
        const dt = last < 0 ? 0.016 : Math.min(0.05, (ts - last) / 1000);
        last = ts;
        if (demo) script(ts);
        step(dt, ts);
        render();
        raf = requestAnimationFrame(frame);
    }
    const ro = new ResizeObserver(function (): void {
        relayout();
    });
    ro.observe(root);
    if (document.fonts) {
        document.fonts.ready.then(function (): void {
            relayout();
        });
    }

    // 시연 계획: 칸마다 포커스 구간(a~b), 타이핑 시작(s)·글자당 ms(rate)·글. 2초 주기로 되풀이하고 주기 첫머리엔 모두 비운다.
    function runPlan(ts: number): void {
        const t = ts % 2000;
        let changed = false;
        for (let i = 0; i < PLAN.length; i += 1) {
            const p = PLAN[i];
            const f = fields[i];
            const isOn = t >= p.a && t < p.b;
            const n = t < p.s ? 0 : Math.min(p.text.length, Math.floor((t - p.s) / p.rate) + 1);
            const v = p.text.slice(0, n);
            if (f.input.value !== v) {
                f.input.value = v;
                changed = true;
            }
            if (f.focus !== isOn) {
                f.focus = isOn;
                changed = true;
                if (!isOn && leaveHook) leaveHook(i);
            }
        }
        if (changed) retarget();
    }
    function resetFields(): void {
        for (let i = 0; i < fields.length; i += 1) {
            fields[i].input.value = "";
            fields[i].focus = false;
        }
        retarget();
    }
    function wire(): void {
        fields.forEach(function (f: F, i: number): void {
            on(f.input, "focus", function (): void {
                stopDemo();
                f.focus = true;
                retarget();
            });
            on(f.input, "blur", function (): void {
                f.focus = false;
                if (leaveHook) leaveHook(i);
                retarget();
            });
            on(f.input, "input", function (): void {
                retarget();
            });
        });
    }

    // 기법: 캐럿 앞 글자의 폭을 거울 span 으로 재서 밑줄 빛 점이 그 x 로 지수 접근한다. 타이핑이 빠르면 점이 뒤처진다.
    const LERP = 0.18;
    const UNDERLINE_H = 4;
    const CARET_OFFSET = 1;
    const TRAIL_FADE = 0.45;
    const INPUT_PAD = 12;
    const RISE_PX = 15;
    const LABEL_SCALE = 0.74;
    const DOT_W = 44;
    const fields: F[] = qa("[data-r=box]").map(function (box: HTMLElement): F {
        const trail = box.querySelector("[data-r=trail]") as HTMLElement;
        const dot = box.querySelector("[data-r=dot]") as HTMLElement;
        trail.style.height = UNDERLINE_H + "px";
        dot.style.height = UNDERLINE_H + "px";
        dot.style.width = DOT_W + "px";
        return {
            box: box,
            input: box.querySelector("input") as HTMLInputElement,
            label: box.querySelector("label") as HTMLElement,
            mirror: box.querySelector("[data-r=mirror]") as HTMLElement,
            trail: trail, dot: dot,
            focus: false, up: false, p: 0, x: INPUT_PAD, tx: INPUT_PAD, w: 400
        };
    });
    const PLAN: Plan[] = [
        { a: 200, b: 1150, s: 250, rate: 60, text: "[email protected]" },
        { a: 1150, b: 1980, s: 1250, rate: 90, text: "Mina Seo" }
    ];
    // 주의: type="email" 칸은 selectionStart 가 null 이고 setSelectionRange 가 예외를 던진다. 그래서 이메일 칸도 type="text" + inputmode="email" 로 둔다.
    // 캐럿 x = 안쪽 여백 + 캐럿 앞 글자 폭 + 보정. 글자 폭은 input 과 같은 글꼴의 거울 span 으로 잰다.
    function caretX(f: F): number {
        const at = f.input.selectionStart;
        const n = at === null ? f.input.value.length : at;
        f.mirror.textContent = f.input.value.slice(0, n);
        return INPUT_PAD + f.mirror.getBoundingClientRect().width + CARET_OFFSET;
    }
    function retarget(): void {
        fields.forEach(function (f: F): void {
            f.up = f.focus || isFilled(f.input);
            if (f.up) f.box.setAttribute("data-up", "");
            else f.box.removeAttribute("data-up");
            f.tx = caretX(f);
            if (reduce) {
                f.p = f.up ? 1 : 0;
                f.x = f.tx;
            }
        });
        if (reduce) render();
    }
    function relayout(): void {
        fields.forEach(function (f: F): void {
            f.w = f.box.clientWidth;
        });
        retarget();
    }
    let jumped = false;
    function script(ts: number): void {
        runPlan(ts);
        const t = ts % 2000;
        const f = fields[0];
        const want = t >= 1040 && t < 1150;
        if (want !== jumped) {
            jumped = want;
            const to = want ? 3 : f.input.value.length;
            f.input.setSelectionRange(to, to);
            retarget();
        }
    }
    function step(dt: number, ts: number): void {
        const fr = dt * 60;
        fields.forEach(function (f: F): void {
            f.p = approach(f.p, f.up ? 1 : 0, 0.22, fr);
            f.x = approach(f.x, f.tx, LERP, fr);
        });
    }
    function render(): void {
        fields.forEach(function (f: F): void {
            const s = 1 - (1 - LABEL_SCALE) * f.p;
            f.label.style.transform = "translate3d(0," + (-RISE_PX * f.p).toFixed(2) + "px,0) scale(" + s.toFixed(3) + ")";
            f.box.style.setProperty("--p", f.p.toFixed(3));
            const lag = Math.abs(f.tx - f.x);
            const glow = clamp(TRAIL_FADE + lag / 80, 0, 1) * f.p;
            f.trail.style.transform = "scaleX(" + clamp(f.x / f.w, 0, 1).toFixed(4) + ")";
            f.trail.style.opacity = glow.toFixed(3);
            f.dot.style.transform = "translate3d(" + (f.x - DOT_W / 2).toFixed(2) + "px,0,0)";
            f.dot.style.opacity = f.p.toFixed(3);
        });
    }
    wire();

    relayout();
    if (reduce) {
        render();
    } else {
        raf = requestAnimationFrame(frame);
    }

    return () => {
      cancelAnimationFrame(raf);
      offs.forEach((f) => f());
      ro.disconnect();
    };
  }, []);

  return (
    <main className={styles["stage"]}>
      <div ref={rootRef} className={styles["fl"] + " is-demo"} data-demo="1">
        <div className={styles["fl__field"]}>
          <div className={styles["fl__box"]} data-r="box">
            <input className={styles["fl__input"]} id="f1" data-r="input" type="text" autoComplete="email" inputMode="email" />
            <label className={styles["fl__label"]} htmlFor="f1" data-r="label">이메일 주소</label>
            <span className={styles["fl__mirror"]} data-r="mirror" aria-hidden="true" />
            <i className={styles["fl__line"]} aria-hidden="true" />
            <i className={styles["fl__trail"]} data-r="trail" aria-hidden="true" />
            <i className={styles["fl__dot"]} data-r="dot" aria-hidden="true" />
          </div>
        </div>
        <div className={styles["fl__field"]}>
          <div className={styles["fl__box"]} data-r="box">
            <input className={styles["fl__input"]} id="f2" data-r="input" type="text" autoComplete="name" />
            <label className={styles["fl__label"]} htmlFor="f2" data-r="label">이름</label>
            <span className={styles["fl__mirror"]} data-r="mirror" aria-hidden="true" />
            <i className={styles["fl__line"]} aria-hidden="true" />
            <i className={styles["fl__trail"]} data-r="trail" aria-hidden="true" />
            <i className={styles["fl__dot"]} data-r="dot" aria-hidden="true" />
          </div>
        </div>
      </div>
    </main>
  );
}