02行の高さの倍数に吸い付くテキストエリアのつまみ

<main class="stage">
<div class="rh is-demo" id="rh" data-demo="1">
  <div class="rh__box" data-r="box">
    <div class="rh__lines">
      <i class="rh__ln"></i><i class="rh__ln"></i><i class="rh__ln"></i><i class="rh__ln"></i><i class="rh__ln"></i><i class="rh__ln"></i><i class="rh__ln"></i><i class="rh__ln"></i>
    </div>
    <button class="rh__grip" data-r="grip" type="button" aria-label="サイズ変更つまみ"></button>
  </div>
  <i class="rh__raw" data-r="raw"></i>
  <p class="rh__rows"><b data-r="rows">0</b><span class="rh__tag">ROWS</span></p>
  <i class="rh__cursor" data-r="cursor"></i>
</div>
</main>

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

// 핵심: 상자 높이는 raw 높이를 줄 높이로 나눈 몫(반올림)에 맞춰 지수 감쇠로 이동한다.

@include stage($stage-paper);

.rh {
  position: relative;
  box-sizing: border-box;
  width: min(100%, 440px);
  height: min(252px, calc(100vh - 2 * clamp(8px, 4vw, 24px)));
  color: $subject-ink;
  font-size: 12px;
  line-height: 16px;
  font-weight: 600;
  user-select: none;
}
.rh b { font-weight: 800; font-variant-numeric: tabular-nums; }
.rh__tag { color: rgba($subject-ink, .72); letter-spacing: .5px; }
.rh__cursor {
  position: absolute;
  left: -10px;
  top: -10px;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background: $subject-cream;
  box-shadow: 0 0 0 3px rgba($subject-ink, .55), $shadow-press;
  opacity: 0;
  pointer-events: none;
  z-index: 6;
}
.rh[data-demo] .rh__cursor { opacity: 1; }
.rh button:focus-visible { outline: 3px solid $subject-ink; outline-offset: 2px; }
@media (prefers-reduced-motion: reduce) {
  .rh__cursor { display: none; }
}

// 점선(raw)과 상자 가장자리가 어긋나 있다가 줄 수가 바뀌는 순간 붙는다.
.rh { color: $subject-ink; }
.rh__box {
  position: absolute;
  left: 0;
  top: 0;
  width: calc(100% - 64px);
  height: 100px;
}
.rh__lines {
  position: absolute;
  inset: 0;
  overflow: hidden;
  border-radius: $r-card;
  background: $subject-mint;
  box-shadow: 0 8px 16px rgba($subject-ink, .2);
}
.rh__ln {
  position: absolute;
  left: $sp-3;
  height: 6px;
  border-radius: $r-xs;
  background: rgba($subject-ink, .55);
}
$ln-widths: 86%, 70%, 90%, 62%, 78%, 88%, 66%, 82%;
@for $i from 0 through 7 {
  .rh__ln:nth-child(#{$i + 1}) {
    top: calc(8px + #{$i} * var(--lh) + (var(--lh) - 6px) / 2);
    width: calc(#{nth($ln-widths, $i + 1)} - 24px);
  }
}
.rh__grip {
  position: absolute;
  left: 50%;
  bottom: -8px;
  width: 64px;
  height: 16px;
  padding: 0;
  border: 0;
  border-radius: $r-pill;
  background: $subject-ink;
  box-shadow: inset 0 0 0 2px $subject-mint;
  transform: translateX(-50%);
  cursor: ns-resize;
  touch-action: none;
}
.rh__raw {
  position: absolute;
  left: 0;
  top: 0;
  width: calc(100% - 64px);
  height: 0;
  border-top: 2px dashed $subject-pink;
  pointer-events: none;
}
.rh__rows {
  position: absolute;
  right: 0;
  top: 0;
  width: 56px;
  margin: 0;
  display: grid;
  justify-items: center;
}
.rh__rows b { font-size: 32px; line-height: 36px; font-weight: 800; }

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);
}

.rh {
  position: relative;
  box-sizing: border-box;
  width: min(100%, 440px);
  height: min(252px, 100vh - 2 * clamp(8px, 4vw, 24px));
  color: #17141a;
  font-size: 12px;
  line-height: 16px;
  font-weight: 600;
  user-select: none;
}

.rh b {
  font-weight: 800;
  font-variant-numeric: tabular-nums;
}

.rh__tag {
  color: rgba(23, 20, 26, 0.72);
  letter-spacing: 0.5px;
}

.rh__cursor {
  position: absolute;
  left: -10px;
  top: -10px;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background: #fff7e6;
  box-shadow: 0 0 0 3px rgba(23, 20, 26, 0.55), 0 6px 12px rgba(23, 20, 26, 0.32);
  opacity: 0;
  pointer-events: none;
  z-index: 6;
}

.rh[data-demo] .rh__cursor {
  opacity: 1;
}

.rh button:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 2px;
}

@media (prefers-reduced-motion: reduce) {
  .rh__cursor {
    display: none;
  }
}
.rh {
  color: #17141a;
}

.rh__box {
  position: absolute;
  left: 0;
  top: 0;
  width: calc(100% - 64px);
  height: 100px;
}

.rh__lines {
  position: absolute;
  inset: 0;
  overflow: hidden;
  border-radius: 16px;
  background: #4cd4a6;
  box-shadow: 0 8px 16px rgba(23, 20, 26, 0.2);
}

.rh__ln {
  position: absolute;
  left: 12px;
  height: 6px;
  border-radius: 4px;
  background: rgba(23, 20, 26, 0.55);
}

.rh__ln:nth-child(1) {
  top: calc(8px + 0 * var(--lh) + (var(--lh) - 6px) / 2);
  width: calc(86% - 24px);
}

.rh__ln:nth-child(2) {
  top: calc(8px + 1 * var(--lh) + (var(--lh) - 6px) / 2);
  width: calc(70% - 24px);
}

.rh__ln:nth-child(3) {
  top: calc(8px + 2 * var(--lh) + (var(--lh) - 6px) / 2);
  width: calc(90% - 24px);
}

.rh__ln:nth-child(4) {
  top: calc(8px + 3 * var(--lh) + (var(--lh) - 6px) / 2);
  width: calc(62% - 24px);
}

.rh__ln:nth-child(5) {
  top: calc(8px + 4 * var(--lh) + (var(--lh) - 6px) / 2);
  width: calc(78% - 24px);
}

.rh__ln:nth-child(6) {
  top: calc(8px + 5 * var(--lh) + (var(--lh) - 6px) / 2);
  width: calc(88% - 24px);
}

.rh__ln:nth-child(7) {
  top: calc(8px + 6 * var(--lh) + (var(--lh) - 6px) / 2);
  width: calc(66% - 24px);
}

.rh__ln:nth-child(8) {
  top: calc(8px + 7 * var(--lh) + (var(--lh) - 6px) / 2);
  width: calc(82% - 24px);
}

.rh__grip {
  position: absolute;
  left: 50%;
  bottom: -8px;
  width: 64px;
  height: 16px;
  padding: 0;
  border: 0;
  border-radius: 999px;
  background: #17141a;
  box-shadow: inset 0 0 0 2px #4cd4a6;
  transform: translateX(-50%);
  cursor: ns-resize;
  touch-action: none;
}

.rh__raw {
  position: absolute;
  left: 0;
  top: 0;
  width: calc(100% - 64px);
  height: 0;
  border-top: 2px dashed #ff9ebb;
  pointer-events: none;
}

.rh__rows {
  position: absolute;
  right: 0;
  top: 0;
  width: 56px;
  margin: 0;
  display: grid;
  justify-items: center;
}

.rh__rows b {
  font-size: 32px;
  line-height: 36px;
  font-weight: 800;
}

(function () {
    const root = document.getElementById("rh");
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let raf = 0;
    let t0 = -1;
    let last = -1;
    const offs = [];
    function q(sel) {
        return root.querySelector(sel);
    }
    function clamp(v, lo, hi) {
        return Math.max(lo, Math.min(hi, v));
    }
    function seg(t, a, b) {
        const c = clamp((t - a) / (b - a), 0, 1);
        return c * c * (3 - 2 * c);
    }
    function on(el, type, fn) {
        el.addEventListener(type, fn);
        offs.push(() => el.removeEventListener(type, fn));
    }
    function stopDemo() {
        demo = false;
        root.removeAttribute("data-demo");
        root.classList.remove("is-demo");
    }
    function pos(e) {
        const r = root.getBoundingClientRect();
        return { x: e.clientX - r.left, y: e.clientY - r.top };
    }
    function spring(x, v, to, k, c, dt) {
        const n = Math.max(1, Math.ceil(dt / 0.008));
        const h = dt / n;
        for (let i = 0; i < n; i += 1) {
            v += (-k * (x - to) - c * v) * h;
            x += v * h;
        }
        return [x, v];
    }
    function sync() {
        if (reduce) {
            snap();
            render();
        }
    }
    // 기법: 끄는 높이(raw)를 줄 높이로 나눠 가장 가까운 줄 수로 반올림하고, 상자는 그 높이까지 지수 감쇠로 끌려간다.
    // 점선은 손이 실제로 있는 높이, 상자는 줄에 맞춘 높이 - 둘의 차이가 계산이다.
    const LINE_H = 28;
    const PAD = 8;
    const ROUND_ROWS = 1;
    const EASE_TO = 14;
    const box = q("[data-r=box]");
    const grip = q("[data-r=grip]");
    const raw = q("[data-r=raw]");
    const rowsEl = q("[data-r=rows]");
    const cursor = q("[data-r=cursor]");
    let h = 0;
    let rawH = 0;
    let rows = 3;
    let px = 0;
    let py = 0;
    let gy = 0;
    let dragging = false;
    let ready = false;
    root.style.setProperty("--lh", LINE_H + "px");
    function maxRows() {
        return Math.max(2, Math.floor((root.clientHeight - 2 * PAD - 12) / LINE_H));
    }
    function rowsFor(height) {
        const r = Math.round((height - 2 * PAD) / LINE_H / ROUND_ROWS) * ROUND_ROWS;
        return clamp(r, 1, maxRows());
    }
    function init() {
        if (ready)
            return;
        ready = true;
        rows = Math.max(2, Math.round(maxRows() * 0.3));
        h = rawH = rows * LINE_H + 2 * PAD;
    }
    function userTakes() {
        stopDemo();
        dragging = false;
        init();
    }
    function script(now) {
        const ph = now % 2000;
        const lo = rows0() * LINE_H + 2 * PAD - 0.4 * LINE_H;
        const hi = maxRows() * LINE_H + 2 * PAD;
        const mid = Math.round(maxRows() * 0.5) * LINE_H + 2 * PAD + 0.45 * LINE_H;
        let y = lo;
        if (ph >= 200 && ph < 900)
            y = lo + (hi - lo) * seg(ph, 200, 900);
        else if (ph >= 900 && ph < 1100)
            y = hi;
        else if (ph >= 1100 && ph < 1800)
            y = hi + (mid - hi) * seg(ph, 1100, 1800);
        else if (ph >= 1800)
            y = mid + (lo - mid) * seg(ph, 1800, 2000);
        py = y - 4;
        gy = 4;
        dragging = ph >= 200;
    }
    function rows0() {
        return Math.max(2, Math.round(maxRows() * 0.3));
    }
    function snap() {
        init();
        h = rows * LINE_H + 2 * PAD;
    }
    function step(dt, now) {
        init();
        if (demo)
            script(now);
        if (dragging) {
            rawH = clamp(py + gy, LINE_H + 2 * PAD, root.clientHeight - 12);
            rows = rowsFor(rawH);
        }
        h += (rows * LINE_H + 2 * PAD - h) * (1 - Math.exp(-EASE_TO * dt));
    }
    function render() {
        box.style.height = h.toFixed(1) + "px";
        raw.style.transform = "translateY(" + rawH.toFixed(1) + "px)";
        rowsEl.textContent = String(rows);
        cursor.style.transform = "translate(" + (root.clientWidth * 0.5 - 32).toFixed(1) + "px," + py.toFixed(1) + "px)";
    }
    on(grip, "pointerdown", (e) => {
        userTakes();
        grip.setPointerCapture(e.pointerId);
        const p = pos(e);
        py = p.y;
        gy = h - p.y;
        dragging = true;
    });
    on(grip, "pointermove", (e) => {
        if (!dragging)
            return;
        py = pos(e).y;
        rawH = clamp(py + gy, LINE_H + 2 * PAD, root.clientHeight - 12);
        rows = rowsFor(rawH);
        sync();
    });
    on(grip, "pointerup", () => { dragging = false; });
    on(grip, "pointercancel", () => { dragging = false; });
    on(grip, "keydown", (e) => {
        if (e.key !== "ArrowUp" && e.key !== "ArrowDown")
            return;
        e.preventDefault();
        userTakes();
        rows = clamp(rows + (e.key === "ArrowDown" ? 1 : -1), 1, maxRows());
        rawH = rows * LINE_H + 2 * PAD;
        sync();
    });
    function frame(ts) {
        if (t0 < 0) {
            t0 = ts;
            last = ts;
        }
        const dt = Math.min(0.05, (ts - last) / 1000);
        last = ts;
        step(dt, ts - t0);
        render();
        raf = requestAnimationFrame(frame);
    }
    if (reduce) {
        stopDemo();
        snap();
        render();
    }
    else {
        raf = requestAnimationFrame(frame);
    }
    return () => {
        cancelAnimationFrame(raf);
        offs.forEach((f) => f());
    };
})();

// LineSnapTextarea - vanilla 판(02-rh-line-snap-textarea) 의 React 포트. 같은 계산을 useEffect 안에서 돌린다.
import { useEffect, useRef } from "react";
import styles from "./LineSnapTextarea.module.scss";

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

  useEffect(() => {
    const rootEl = rootRef.current;
    if (!rootEl) return;
    const root: HTMLElement = rootEl;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let raf = 0;
    let t0 = -1;
    let last = -1;
    const offs: Array<() => void> = [];

    function q<T extends Element>(sel: string): T {
      return root.querySelector(sel) as T;
    }
    function clamp(v: number, lo: number, hi: number): number {
      return Math.max(lo, Math.min(hi, v));
    }
    function seg(t: number, a: number, b: number): number {
      const c = clamp((t - a) / (b - a), 0, 1);
      return c * c * (3 - 2 * c);
    }
    function on(el: EventTarget, type: string, fn: (e: any) => void): void {
      el.addEventListener(type, fn);
      offs.push(() => el.removeEventListener(type, fn));
    }
    function stopDemo(): void {
      demo = false;
      root.removeAttribute("data-demo");
      root.classList.remove("is-demo");
    }
    function pos(e: PointerEvent): { x: number; y: number } {
      const r = root.getBoundingClientRect();
      return { x: e.clientX - r.left, y: e.clientY - r.top };
    }
    function spring(x: number, v: number, to: number, k: number, c: number, dt: number): number[] {
      const n = Math.max(1, Math.ceil(dt / 0.008));
      const h = dt / n;
      for (let i = 0; i < n; i += 1) {
        v += (-k * (x - to) - c * v) * h;
        x += v * h;
      }
      return [x, v];
    }
    function sync(): void {
      if (reduce) {
        snap();
        render();
      }
    }

    // 기법: 끄는 높이(raw)를 줄 높이로 나눠 가장 가까운 줄 수로 반올림하고, 상자는 그 높이까지 지수 감쇠로 끌려간다.
    // 점선은 손이 실제로 있는 높이, 상자는 줄에 맞춘 높이 - 둘의 차이가 계산이다.
    const LINE_H = 28;
    const PAD = 8;
    const ROUND_ROWS = 1;
    const EASE_TO = 14;
    const box = q<HTMLElement>("[data-r=box]");
    const grip = q<HTMLElement>("[data-r=grip]");
    const raw = q<HTMLElement>("[data-r=raw]");
    const rowsEl = q<HTMLElement>("[data-r=rows]");
    const cursor = q<HTMLElement>("[data-r=cursor]");
    let h = 0;
    let rawH = 0;
    let rows = 3;
    let px = 0;
    let py = 0;
    let gy = 0;
    let dragging = false;
    let ready = false;
    root.style.setProperty("--lh", LINE_H + "px");

    function maxRows(): number {
      return Math.max(2, Math.floor((root.clientHeight - 2 * PAD - 12) / LINE_H));
    }

    function rowsFor(height: number): number {
      const r = Math.round((height - 2 * PAD) / LINE_H / ROUND_ROWS) * ROUND_ROWS;
      return clamp(r, 1, maxRows());
    }

    function init(): void {
      if (ready) return;
      ready = true;
      rows = Math.max(2, Math.round(maxRows() * 0.3));
      h = rawH = rows * LINE_H + 2 * PAD;
    }

    function userTakes(): void {
      stopDemo();
      dragging = false;
      init();
    }

    function script(now: number): void {
      const ph = now % 2000;
      const lo = rows0() * LINE_H + 2 * PAD - 0.4 * LINE_H;
      const hi = maxRows() * LINE_H + 2 * PAD;
      const mid = Math.round(maxRows() * 0.5) * LINE_H + 2 * PAD + 0.45 * LINE_H;
      let y = lo;
      if (ph >= 200 && ph < 900) y = lo + (hi - lo) * seg(ph, 200, 900);
      else if (ph >= 900 && ph < 1100) y = hi;
      else if (ph >= 1100 && ph < 1800) y = hi + (mid - hi) * seg(ph, 1100, 1800);
      else if (ph >= 1800) y = mid + (lo - mid) * seg(ph, 1800, 2000);
      py = y - 4;
      gy = 4;
      dragging = ph >= 200;
    }

    function rows0(): number {
      return Math.max(2, Math.round(maxRows() * 0.3));
    }

    function snap(): void {
      init();
      h = rows * LINE_H + 2 * PAD;
    }

    function step(dt: number, now: number): void {
      init();
      if (demo) script(now);
      if (dragging) {
        rawH = clamp(py + gy, LINE_H + 2 * PAD, root.clientHeight - 12);
        rows = rowsFor(rawH);
      }
      h += (rows * LINE_H + 2 * PAD - h) * (1 - Math.exp(-EASE_TO * dt));
    }

    function render(): void {
      box.style.height = h.toFixed(1) + "px";
      raw.style.transform = "translateY(" + rawH.toFixed(1) + "px)";
      rowsEl.textContent = String(rows);
      cursor.style.transform = "translate(" + (root.clientWidth * 0.5 - 32).toFixed(1) + "px," + py.toFixed(1) + "px)";
    }

    on(grip, "pointerdown", (e: PointerEvent) => {
      userTakes();
      grip.setPointerCapture(e.pointerId);
      const p = pos(e);
      py = p.y;
      gy = h - p.y;
      dragging = true;
    });
    on(grip, "pointermove", (e: PointerEvent) => {
      if (!dragging) return;
      py = pos(e).y;
      rawH = clamp(py + gy, LINE_H + 2 * PAD, root.clientHeight - 12);
      rows = rowsFor(rawH);
      sync();
    });
    on(grip, "pointerup", () => { dragging = false; });
    on(grip, "pointercancel", () => { dragging = false; });
    on(grip, "keydown", (e: KeyboardEvent) => {
      if (e.key !== "ArrowUp" && e.key !== "ArrowDown") return;
      e.preventDefault();
      userTakes();
      rows = clamp(rows + (e.key === "ArrowDown" ? 1 : -1), 1, maxRows());
      rawH = rows * LINE_H + 2 * PAD;
      sync();
    });

    function frame(ts: number): void {
      if (t0 < 0) {
        t0 = ts;
        last = ts;
      }
      const dt = Math.min(0.05, (ts - last) / 1000);
      last = ts;
      step(dt, ts - t0);
      render();
      raf = requestAnimationFrame(frame);
    }
    if (reduce) {
      stopDemo();
      snap();
      render();
    } else {
      raf = requestAnimationFrame(frame);
    }
    return () => {
      cancelAnimationFrame(raf);
      offs.forEach((f) => f());
    };

  }, []);

  return (
    <main className={styles["stage"]}>
      <div ref={rootRef} className={styles["rh"] + " is-demo"} data-demo="1">
        <div className={styles["rh__box"]} data-r="box">
          <div className={styles["rh__lines"]}>
            <i className={styles["rh__ln"]}></i><i className={styles["rh__ln"]}></i><i className={styles["rh__ln"]}></i><i className={styles["rh__ln"]}></i><i className={styles["rh__ln"]}></i><i className={styles["rh__ln"]}></i><i className={styles["rh__ln"]}></i><i className={styles["rh__ln"]}></i>
          </div>
          <button className={styles["rh__grip"]} data-r="grip" type="button" aria-label="크기 조절 손잡이"></button>
        </div>
        <i className={styles["rh__raw"]} data-r="raw"></i>
        <p className={styles["rh__rows"]}><b data-r="rows">0</b><span className={styles["rh__tag"]}>ROWS</span></p>
        <i className={styles["rh__cursor"]} data-r="cursor"></i>
      </div>
    </main>
  );
}