kit-ui/components/BackToTop.tsx

'use client';

import { useState, useEffect, useRef } from 'react';
import { ChevronUp } from 'lucide-react';

export default function BackToTop() {
  const [isVisible, setIsVisible] = useState(false);
  const barRef = useRef<HTMLDivElement>(null);

  useEffect(() => {
    const onScroll = () => {
      setIsVisible(window.scrollY > 300);
      if (barRef.current) {
        const el = document.documentElement;
        const scrolled = el.scrollTop / (el.scrollHeight - el.clientHeight);
        barRef.current.style.transform = `scaleX(${scrolled})`;
      }
    };
    window.addEventListener('scroll', onScroll, { passive: true });
    return () => window.removeEventListener('scroll', onScroll);
  }, []);

  return (
    <>
      {/* Scroll progress bar */}
      <div
        ref={barRef}
        className="fixed top-0 left-0 right-0 h-px bg-primary z-50 origin-left"
        style={{ transform: 'scaleX(0)', transition: 'transform 0.1s linear' }}
      />

      {/* Back to top button */}
      {isVisible && (
        <button
          onClick={() => window.scrollTo({ top: 0, behavior: 'smooth' })}
          className="fixed bottom-6 right-6 w-8 h-8 glass rounded-lg flex items-center justify-center text-muted-foreground/40 hover:text-primary hover:border-primary/40 transition-all duration-200 z-40"
          aria-label="Back to top"
          style={{ animation: 'fadeIn 0.2s ease-out both' }}
        >
          <ChevronUp className="w-3.5 h-3.5" />
        </button>
      )}
    </>
  );
}