aragrp/components/scroll-to-top.tsx

"use client";

import { useEffect, useState } from "react";
import { ChevronUp } from "lucide-react";
import { Button } from "@/components/ui/button";
import { cn } from "@/lib/utils";

const SCROLL_THRESHOLD = 0.4; // show after 40% scroll

export function ScrollToTop({ className }: { className?: string }) {
  const [visible, setVisible] = useState(false);

  useEffect(() => {
    function onScroll() {
      const maxScroll =
        document.documentElement.scrollHeight - window.innerHeight;
      if (maxScroll <= 0) {
        setVisible(false);
        return;
      }
      const ratio = window.scrollY / maxScroll;
      setVisible(ratio >= SCROLL_THRESHOLD);
    }

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

  const scrollToTop = () => {
    window.scrollTo({ top: 0, behavior: "smooth" });
  };

  if (!visible) return null;

  return (
    <div
      className={cn(
        "fixed bottom-6 start-6 z-50 transition-opacity duration-300",
        className
      )}
    >
      <Button
        size="icon-lg"
        variant="secondary"
        onClick={scrollToTop}
        aria-label="برگشت به بالا"
      >
        <ChevronUp />
      </Button>
    </div>
  );
}