04An Arrow That Keeps Pointing at the Anchor While the Body Shifts

<main class="stage">
<div class="pm is-demo" id="pm" data-demo="1">
  <p class="pm__live" data-r="live" aria-live="off" aria-atomic="true"></p>
  <i class="pm__line" data-r="line"></i>
  <div class="pm__pop" data-r="pop"><i class="pm__arrow" data-r="arrow"></i><span class="pm__txt">Check the email address</span></div>
  <button class="pm__anchor" data-r="anchor" type="button" aria-label="Anchor, move with left and right keys"></button>
</div>
</main>

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

// 핵심: 팝오버 몸통은 가장자리에 막혀 밀리지만 화살표 끝은 앵커 중심을 보간으로 쫓아가 항상 앵커를 가리킨다.

@include stage($stage-paper);

.pm {
  position: relative;
  box-sizing: border-box;
  overflow: hidden;
  width: min(100%, 440px);
  height: min(252px, calc(100vh - 2 * clamp(8px, 4vw, 24px)));
  border-radius: $r-card;
  background: rgba($subject-ink, .07);
  color: $subject-ink;
  font-size: 12px;
  line-height: 16px;
  font-weight: 600;
  user-select: none;
}
.pm b { font-weight: 800; font-variant-numeric: tabular-nums; }
.pm__live {
  position: absolute;
  left: $sp-3;
  top: $sp-2;
  margin: 0;
  z-index: 9;
  color: rgba($subject-ink, .72);
  font-variant-numeric: tabular-nums;
  white-space: nowrap;
  pointer-events: none;
}
.pm button { font-family: inherit; }
.pm button:focus-visible { outline: 3px solid $subject-ink; outline-offset: 2px; }



.pm { touch-action: none; }
$accent: $subject-lilac;
.pm__line {
  position: absolute;
  left: 0;
  top: 0;
  width: 2px;
  height: 1px;
  transform-origin: 0 0;
  background: rgba($subject-ink, .3);
  pointer-events: none;
}
.pm__anchor {
  position: absolute;
  left: 0;
  top: 0;
  box-sizing: border-box;
  width: 28px;
  height: 28px;
  padding: 0;
  border: 0;
  border-radius: 50%;
  background: $subject-pink;
  box-shadow: 0 0 0 4px rgba($subject-pink, .4), $shadow-press;
  cursor: grab;
  z-index: 5;
}
.pm__pop {
  position: absolute;
  left: 0;
  top: 0;
  box-sizing: border-box;
  display: flex;
  align-items: center;
  width: 168px;
  height: 56px;
  padding: $sp-2 $sp-3;
  border-radius: $r-control;
  background: $accent;
  color: $subject-ink;
  box-shadow: $shadow-raised;
  z-index: 4;
}
.pm__arrow {
  position: absolute;
  left: 0;
  top: 0;
  width: 16px;
  height: 16px;
  border-radius: $r-xs;
  background: $accent;
}
.pm__txt { position: relative; word-break: keep-all; }
@media (prefers-reduced-motion: reduce) {
  .pm__anchor { cursor: default; }
}

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);
}

.pm {
  position: relative;
  box-sizing: border-box;
  overflow: hidden;
  width: min(100%, 440px);
  height: min(252px, 100vh - 2 * clamp(8px, 4vw, 24px));
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.07);
  color: #17141a;
  font-size: 12px;
  line-height: 16px;
  font-weight: 600;
  user-select: none;
}

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

.pm__live {
  position: absolute;
  left: 12px;
  top: 8px;
  margin: 0;
  z-index: 9;
  color: rgba(23, 20, 26, 0.72);
  font-variant-numeric: tabular-nums;
  white-space: nowrap;
  pointer-events: none;
}

.pm button {
  font-family: inherit;
}

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

.pm {
  touch-action: none;
}

.pm__line {
  position: absolute;
  left: 0;
  top: 0;
  width: 2px;
  height: 1px;
  transform-origin: 0 0;
  background: rgba(23, 20, 26, 0.3);
  pointer-events: none;
}

.pm__anchor {
  position: absolute;
  left: 0;
  top: 0;
  box-sizing: border-box;
  width: 28px;
  height: 28px;
  padding: 0;
  border: 0;
  border-radius: 50%;
  background: #ff9ebb;
  box-shadow: 0 0 0 4px rgba(255, 158, 187, 0.4), 0 6px 12px rgba(23, 20, 26, 0.32);
  cursor: grab;
  z-index: 5;
}

.pm__pop {
  position: absolute;
  left: 0;
  top: 0;
  box-sizing: border-box;
  display: flex;
  align-items: center;
  width: 168px;
  height: 56px;
  padding: 8px 12px;
  border-radius: 12px;
  background: #b9a0ff;
  color: #17141a;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  z-index: 4;
}

.pm__arrow {
  position: absolute;
  left: 0;
  top: 0;
  width: 16px;
  height: 16px;
  border-radius: 4px;
  background: #b9a0ff;
}

.pm__txt {
  position: relative;
  word-break: keep-all;
}

@media (prefers-reduced-motion: reduce) {
  .pm__anchor {
    cursor: default;
  }
}

(function () {
    const root = document.getElementById("pm");
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let raf = 0;
    let t0 = -1;
    let last = -1;
    let acc = 0;
    let clock = 0;
    const offs = [];
    const live = root.querySelector("[data-r=live]");
    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() {
        if (!demo)
            return;
        demo = false;
        root.removeAttribute("data-demo");
        root.classList.remove("is-demo");
        live.setAttribute("aria-live", "polite");
    }
    if (reduce)
        live.setAttribute("aria-live", "polite");
    let shown = "";
    function say(s) {
        if (s === shown)
            return;
        shown = s;
        live.textContent = s;
    }
    function makeRng(seed) {
        let s = seed >>> 0;
        return () => {
            s = (Math.imul(s, 1664525) + 1013904223) >>> 0;
            return s / 4294967296;
        };
    }
    function local(e) {
        const r = root.getBoundingClientRect();
        return { x: e.clientX - r.left, y: e.clientY - r.top };
    }
    function sync() {
        if (reduce) {
            snap();
            render();
        }
    }
    // 기법: 몸통은 EDGE_PAD 에서 막혀 앵커보다 최대 SHIFT_MAX 만큼 덜 가고, 화살표 끝은 따로 ARROW_LERP 로 앵커 중심을 쫓는다. 꼬리와 몸통이 서로 다른 속도로 움직인다.
    const ARROW_LERP = 0.15;
    const EDGE_PAD = 8;
    const ARROW_SIZE = 8;
    const SHIFT_MAX = 72;
    const BODY_W = 168;
    const BODY_H = 56;
    const BODY_LERP = 0.35;
    const ANCHOR_R = 14;
    const GAP = 12;
    const SWEEP_MS = 2400;
    const anchor = q("[data-r=anchor]");
    const pop = q("[data-r=pop]");
    const arrow = q("[data-r=arrow]");
    const line = q("[data-r=line]");
    let nx = reduce ? 0.94 : 0.5;
    let bodyX = 0;
    let tip = 0;
    let placed = false;
    let dragging = false;
    let shift = 0;
    function axOf(w) {
        return clamp(nx * w, EDGE_PAD + BODY_W / 2 - SHIFT_MAX, w - EDGE_PAD - BODY_W / 2 + SHIFT_MAX);
    }
    function bodyTarget(w, ax) {
        const desired = ax - BODY_W / 2;
        const clamped = clamp(desired, EDGE_PAD, w - EDGE_PAD - BODY_W);
        shift = clamp(clamped - desired, -SHIFT_MAX, SHIFT_MAX);
        return desired + shift;
    }
    function tick(ms) {
        if (demo)
            nx = 0.5 + 0.47 * Math.sin((Math.max(0, ms - 200) / SWEEP_MS) * Math.PI * 2);
        const w = root.clientWidth;
        const ax = axOf(w);
        if (!placed)
            snap();
        bodyX += (bodyTarget(w, ax) - bodyX) * BODY_LERP;
        tip += (ax - tip) * ARROW_LERP;
    }
    function snap() {
        const w = root.clientWidth;
        const ax = axOf(w);
        bodyX = bodyTarget(w, ax);
        tip = ax;
        placed = true;
    }
    function render() {
        const w = root.clientWidth;
        const h = root.clientHeight;
        const ax = axOf(w);
        const ay = h * 0.3;
        const by = ay + ANCHOR_R + GAP;
        anchor.style.transform = "translate3d(" + (ax - ANCHOR_R).toFixed(1) + "px," + (ay - ANCHOR_R).toFixed(1) + "px,0)";
        pop.style.transform = "translate3d(" + bodyX.toFixed(1) + "px," + by.toFixed(1) + "px,0)";
        const rel = clamp(tip - bodyX, ARROW_SIZE + 6, BODY_W - ARROW_SIZE - 6);
        arrow.style.transform = "translate3d(" + (rel - ARROW_SIZE).toFixed(1) + "px," + (-ARROW_SIZE).toFixed(1) + "px,0) rotate(45deg)";
        line.style.transform = "translate3d(" + (ax - 1).toFixed(1) + "px," + (ay + ANCHOR_R).toFixed(1) + "px,0) scale(1," + (by - ay - ANCHOR_R).toFixed(1) + ")";
        say("shift " + Math.round(shift) + "  gap " + Math.round(Math.abs(tip - ax)));
    }
    function moveTo(e) {
        nx = clamp(local(e).x / root.clientWidth, 0.04, 0.96);
        sync();
    }
    on(root, "pointerdown", (e) => {
        stopDemo();
        dragging = true;
        root.setPointerCapture(e.pointerId);
        moveTo(e);
    });
    on(root, "pointermove", (e) => {
        if (dragging)
            moveTo(e);
    });
    on(root, "pointerup", () => {
        dragging = false;
    });
    on(root, "pointercancel", () => {
        dragging = false;
    });
    on(anchor, "keydown", (e) => {
        const dx = e.key === "ArrowRight" ? 1 : e.key === "ArrowLeft" ? -1 : 0;
        if (dx === 0)
            return;
        e.preventDefault();
        stopDemo();
        nx = clamp(nx + dx * 0.06, 0.04, 0.96);
        sync();
    });
    snap();
    const FRAME_MS = 1000 / 60;
    function frame(ts) {
        if (t0 < 0) {
            t0 = ts;
            last = ts;
        }
        acc += Math.min(100, ts - last);
        last = ts;
        let n = 0;
        while (acc >= FRAME_MS && n < 6) {
            clock += FRAME_MS;
            tick(clock);
            acc -= FRAME_MS;
            n += 1;
        }
        if (n === 6)
            acc = 0;
        render();
        raf = requestAnimationFrame(frame);
    }
    const ro = new ResizeObserver(() => sync());
    ro.observe(root);
    if (reduce) {
        stopDemo();
        snap();
        render();
    }
    else {
        raf = requestAnimationFrame(frame);
    }
    return () => {
        cancelAnimationFrame(raf);
        offs.forEach((f) => f());
        ro.disconnect();
    };
})();

// ArrowTracksAnchor - vanilla 판(04-pm-arrow-tracks-anchor) 의 React 포트. 같은 계산을 useEffect 안에서 돌린다.
import { useEffect, useRef } from "react";
import styles from "./ArrowTracksAnchor.module.scss";

export default function ArrowTracksAnchor() {
  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;
    let acc = 0;
    let clock = 0;
    const offs: Array<() => void> = [];
    const live = root.querySelector("[data-r=live]") as HTMLElement;

    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 {
      if (!demo) return;
      demo = false;
      root.removeAttribute("data-demo");
      root.classList.remove("is-demo");
      live.setAttribute("aria-live", "polite");
    }
    if (reduce) live.setAttribute("aria-live", "polite");
    let shown = "";
    function say(s: string): void {
      if (s === shown) return;
      shown = s;
      live.textContent = s;
    }
    function makeRng(seed: number): () => number {
      let s = seed >>> 0;
      return () => {
        s = (Math.imul(s, 1664525) + 1013904223) >>> 0;
        return s / 4294967296;
      };
    }
    function local(e: PointerEvent): { x: number; y: number } {
      const r = root.getBoundingClientRect();
      return { x: e.clientX - r.left, y: e.clientY - r.top };
    }
    function sync(): void {
      if (reduce) {
        snap();
        render();
      }
    }

    // 기법: 몸통은 EDGE_PAD 에서 막혀 앵커보다 최대 SHIFT_MAX 만큼 덜 가고, 화살표 끝은 따로 ARROW_LERP 로 앵커 중심을 쫓는다. 꼬리와 몸통이 서로 다른 속도로 움직인다.
    const ARROW_LERP = 0.15;
    const EDGE_PAD = 8;
    const ARROW_SIZE = 8;
    const SHIFT_MAX = 72;
    const BODY_W = 168;
    const BODY_H = 56;
    const BODY_LERP = 0.35;
    const ANCHOR_R = 14;
    const GAP = 12;
    const SWEEP_MS = 2400;
    const anchor = q<HTMLElement>("[data-r=anchor]");
    const pop = q<HTMLElement>("[data-r=pop]");
    const arrow = q<HTMLElement>("[data-r=arrow]");
    const line = q<HTMLElement>("[data-r=line]");
    let nx = reduce ? 0.94 : 0.5;
    let bodyX = 0;
    let tip = 0;
    let placed = false;
    let dragging = false;
    let shift = 0;

    function axOf(w: number): number {
      return clamp(nx * w, EDGE_PAD + BODY_W / 2 - SHIFT_MAX, w - EDGE_PAD - BODY_W / 2 + SHIFT_MAX);
    }
    function bodyTarget(w: number, ax: number): number {
      const desired = ax - BODY_W / 2;
      const clamped = clamp(desired, EDGE_PAD, w - EDGE_PAD - BODY_W);
      shift = clamp(clamped - desired, -SHIFT_MAX, SHIFT_MAX);
      return desired + shift;
    }
    function tick(ms: number): void {
      if (demo) nx = 0.5 + 0.47 * Math.sin((Math.max(0, ms - 200) / SWEEP_MS) * Math.PI * 2);
      const w = root.clientWidth;
      const ax = axOf(w);
      if (!placed) snap();
      bodyX += (bodyTarget(w, ax) - bodyX) * BODY_LERP;
      tip += (ax - tip) * ARROW_LERP;
    }
    function snap(): void {
      const w = root.clientWidth;
      const ax = axOf(w);
      bodyX = bodyTarget(w, ax);
      tip = ax;
      placed = true;
    }
    function render(): void {
      const w = root.clientWidth;
      const h = root.clientHeight;
      const ax = axOf(w);
      const ay = h * 0.3;
      const by = ay + ANCHOR_R + GAP;
      anchor.style.transform = "translate3d(" + (ax - ANCHOR_R).toFixed(1) + "px," + (ay - ANCHOR_R).toFixed(1) + "px,0)";
      pop.style.transform = "translate3d(" + bodyX.toFixed(1) + "px," + by.toFixed(1) + "px,0)";
      const rel = clamp(tip - bodyX, ARROW_SIZE + 6, BODY_W - ARROW_SIZE - 6);
      arrow.style.transform = "translate3d(" + (rel - ARROW_SIZE).toFixed(1) + "px," + (-ARROW_SIZE).toFixed(1) + "px,0) rotate(45deg)";
      line.style.transform = "translate3d(" + (ax - 1).toFixed(1) + "px," + (ay + ANCHOR_R).toFixed(1) + "px,0) scale(1," + (by - ay - ANCHOR_R).toFixed(1) + ")";
      say("shift " + Math.round(shift) + "  gap " + Math.round(Math.abs(tip - ax)));
    }
    function moveTo(e: PointerEvent): void {
      nx = clamp(local(e).x / root.clientWidth, 0.04, 0.96);
      sync();
    }
    on(root, "pointerdown", (e: PointerEvent) => {
      stopDemo();
      dragging = true;
      root.setPointerCapture(e.pointerId);
      moveTo(e);
    });
    on(root, "pointermove", (e: PointerEvent) => {
      if (dragging) moveTo(e);
    });
    on(root, "pointerup", () => {
      dragging = false;
    });
    on(root, "pointercancel", () => {
      dragging = false;
    });
    on(anchor, "keydown", (e: KeyboardEvent) => {
      const dx = e.key === "ArrowRight" ? 1 : e.key === "ArrowLeft" ? -1 : 0;
      if (dx === 0) return;
      e.preventDefault();
      stopDemo();
      nx = clamp(nx + dx * 0.06, 0.04, 0.96);
      sync();
    });

    snap();

    const FRAME_MS = 1000 / 60;
    function frame(ts: number): void {
      if (t0 < 0) {
        t0 = ts;
        last = ts;
      }
      acc += Math.min(100, ts - last);
      last = ts;
      let n = 0;
      while (acc >= FRAME_MS && n < 6) {
        clock += FRAME_MS;
        tick(clock);
        acc -= FRAME_MS;
        n += 1;
      }
      if (n === 6) acc = 0;
      render();
      raf = requestAnimationFrame(frame);
    }
    const ro = new ResizeObserver(() => sync());
    ro.observe(root);
    if (reduce) {
      stopDemo();
      snap();
      render();
    } else {
      raf = requestAnimationFrame(frame);
    }
    return () => {
      cancelAnimationFrame(raf);
      offs.forEach((f) => f());
      ro.disconnect();
    };

  }, []);

  return (
    <main className={styles["stage"]}>
      <div ref={rootRef} className={styles["pm"] + " is-demo"} data-demo="1">
        <p className={styles["pm__live"]} data-r="live" aria-live="off" aria-atomic="true"></p>
        <i className={styles["pm__line"]} data-r="line"></i>
        <div className={styles["pm__pop"]} data-r="pop"><i className={styles["pm__arrow"]} data-r="arrow"></i><span className={styles["pm__txt"]}>이메일 주소를 확인하세요</span></div>
        <button className={styles["pm__anchor"]} data-r="anchor" type="button" aria-label="앵커 · 좌우 방향키로 옮기기"></button>
      </div>
    </main>
  );
}