09New message auto-scroll pin

<main class="stage">
  <div class="thread">
    <div class="bubble bubble--in">Sounds good</div>
    <div class="bubble bubble--out is-demo">On my way!</div>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────────
$duration: $dur-base;      // 새 메시지가 하단까지 밀려 올라오는 데 걸리는 시간
$easing: $ease-out;
$color: $subject-cream;    // 새 메시지 말풍선 색 (ink 무대라 cream)

@include stage($stage-ink);

.thread {
  display: grid;
  gap: 10px;
  justify-items: end;
  width: min(220px, 78vw);
}
.bubble {
  padding: 12px 18px;
  border-radius: $r-card;
  font-weight: 600; font-size: clamp(13px, 4vw, 15px); line-height: 1.4; font-family: inherit;
  max-width: 80%;
}
.bubble--in {
  justify-self: start;
  background: rgba(255, 255, 255, .1);
  color: #fff;
}
.bubble--out {
  background: $color;
  color: $stage-ink;
  transform-origin: bottom center; // 규칙8: 순수 세로 이동, origin 무의미

  &.is-demo { animation: pin-in-loop $dur-loop $easing infinite; }
}

// 스크롤이 바닥에 붙어 있을 때만 새 메시지가 아래에서 밀려 올라와 자리를 잡는다
@keyframes pin-in-loop {
  0%   { transform: translateY(46px); opacity: 0; }
  28%  { transform: translateY(0); opacity: 1; }
  84%  { transform: translateY(0); opacity: 1; }
  100% { transform: translateY(46px); opacity: 0; }
}

@media (prefers-reduced-motion: reduce) {
  .bubble--out.is-demo { animation: none !important; transform: none; opacity: 1; }
}

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);
}

.thread {
  display: grid;
  gap: 10px;
  justify-items: end;
  width: min(220px, 78vw);
}

.bubble {
  padding: 12px 18px;
  border-radius: 16px;
  font-weight: 600;
  font-size: clamp(13px, 4vw, 15px);
  line-height: 1.4;
  font-family: inherit;
  max-width: 80%;
}

.bubble--in {
  justify-self: start;
  background: rgba(255, 255, 255, 0.1);
  color: #fff;
}

.bubble--out {
  background: #fff7e6;
  color: #17141a;
  transform-origin: bottom center;
}
.bubble--out.is-demo {
  animation: pin-in-loop 2s ease-out infinite;
}

@keyframes pin-in-loop {
  0% {
    transform: translateY(46px);
    opacity: 0;
  }
  28% {
    transform: translateY(0);
    opacity: 1;
  }
  84% {
    transform: translateY(0);
    opacity: 1;
  }
  100% {
    transform: translateY(46px);
    opacity: 0;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bubble--out.is-demo {
    animation: none !important;
    transform: none;
    opacity: 1;
  }
}

import type { CSSProperties, ReactNode } from "react";
import styles from "./ScrollPinNewMessage.module.scss";

export type ScrollPinNewMessageProps = {
  children: ReactNode;
  /** 하단까지 밀려 올라오는 데 걸리는 시간 (기본 300ms) */
  duration?: number;
  easing?: string;
  className?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string };

export default function ScrollPinNewMessage({
  children,
  duration = 300,
  easing = "ease-out",
  className = "",
}: ScrollPinNewMessageProps) {
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing };

  return (
    <div className={`${styles.bubble} ${styles.enter} ${className}`} style={vars}>
      {children}
    </div>
  );
}