/**
 * 回到顶部浮动按钮(前台两页共用,此前 Matches/Standings 各复制一份)。
 * 方角纸片风:去掉早期版本的 rounded-full + shadow-lg,与全站方角
 * 无阴影语言对齐;出现/隐藏仅动画 transform 与 opacity。
 *
 * 滚动监听已抽到 `lib/useScroll` 的 useWindowScrollY —— 全站三处
 * 滚动监听此前各自实现一遍生命周期,现统一为单一来源。
 */
import { useWindowScrollY } from '../lib/useScroll'

export default function BackTop({ threshold = 300 }: { threshold?: number }) {
  const scrollY = useWindowScrollY()
  const show = scrollY > threshold

  return (
    <button
      onClick={() => window.scrollTo({ top: 0, behavior: 'smooth' })}
      className={`fixed bottom-6 right-6 z-40 flex h-10 w-10 items-center justify-center border border-ink-300 bg-paper-50 text-ink-600 transition-all duration-300 hover:border-ink-900 hover:bg-ink-900 hover:text-paper-50 ${
        show ? 'translate-y-0 opacity-100' : 'pointer-events-none translate-y-4 opacity-0'
      }`}
      aria-label="回到顶部"
    >
      <svg className="h-4 w-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
        <path strokeLinecap="round" strokeLinejoin="round" d="M5 15l7-7 7 7" />
      </svg>
    </button>
  )
}