"use client";

import { useEffect, useState } from "react";

export default function BackToTop() {
  const [visible, setVisible] = useState(false);

  useEffect(() => {
    const onScroll = () => setVisible(window.scrollY > 600);
    window.addEventListener("scroll", onScroll, { passive: true });
    return () => window.removeEventListener("scroll", onScroll);
  }, []);

  return (
    <button className={`back-to-top${visible ? " visible" : ""}`}
      onClick={() => window.scrollTo({ top: 0, behavior: "smooth" })}
      aria-label="返回顶部">
      <svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2"><path d="M12 19V5M5 12l7-7 7 7" /></svg>
    </button>
  );
}