"use client";

import { useEffect, useState } from "react";
import { ArrowUp } from "lucide-react";
import { scrollBehavior } from "@/lib/motion";

const SHOW_AFTER = 400;

/**
 * 公开站长页回顶：滚过阈值后右下角浮现。
 * 短页自然不出现；帖子移动端底栏时抬高，避免叠住操作条。
 */
export default function BackToTop({
  liftForMobileBar = false,
}: {
  liftForMobileBar?: boolean;
}) {
  const [visible, setVisible] = useState(false);

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

  if (!visible) return null;

  return (
    <button
      type="button"
      className={`j13-back-top j13-toast-in${liftForMobileBar ? " is-lifted" : ""}`}
      onClick={() => window.scrollTo({ top: 0, behavior: scrollBehavior() })}
      aria-label="回到顶部"
    >
      <ArrowUp size={18} strokeWidth={2.25} aria-hidden />
    </button>
  );
}