import { useEffect, useRef, useState } from "react";
import { createPortal } from "react-dom";
import { ArrowUp } from "lucide-react";
import { useTranslation } from "react-i18next";

// A floating "back to top" button that fades in once the current page's scroll container is
// scrolled past a threshold, and smooth-scrolls it back to the top. Every page scrolls inside a
// <main class="overflow-y-auto"> — the normal pages share App's, the channel page renders its own
// — so this follows whichever one is mounted. Portaled to <body> so its fixed position is
// viewport-relative regardless of transformed ancestors.
export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; threshold?: number }) {
  const { t } = useTranslation();
  const [show, setShow] = useState(false);
  const scrollerRef = useRef<HTMLElement | null>(null);

  // Listen on the document in the CAPTURE phase and read the scroller off the event, rather than
  // looking <main> up when this binds. Scroll events don't bubble, but they do capture — and the
  // lookup was a race the lazy pages lost: on the FIRST visit to the channel page its chunk is
  // still loading, so the Suspense fallback is on screen, there is no <main> to find, and the
  // effect gave up for good (its dep never changes again). The button then stayed dead for that
  // whole visit and came back only once the chunk was cached. Nothing to find, nothing to race.
  useEffect(() => {
    const onScroll = (e: Event) => {
      const el = e.target as HTMLElement | null;
      if (!el || el.tagName !== "MAIN") return;
      scrollerRef.current = el;
      setShow(el.scrollTop > threshold);
    };
    document.addEventListener("scroll", onScroll, { passive: true, capture: true });
    return () => document.removeEventListener("scroll", onScroll, { capture: true });
  }, [threshold]);

  // On navigation, re-read where the new page sits: usually the top (so hide), but some pages
  // restore their scroll position. Missing <main> here is now harmless — a page that hasn't
  // rendered hasn't scrolled either, and the listener above picks it up the moment it does.
  useEffect(() => {
    const el = document.querySelector("main");
    scrollerRef.current = el;
    setShow(!!el && el.scrollTop > threshold);
  }, [dep, threshold]);

  const toTop = () =>
    (scrollerRef.current ?? document.querySelector("main"))?.scrollTo({
      top: 0,
      behavior: "smooth",
    });

  // No aria-label/title attributes: ad-block "annoyance" cosmetic filters (e.g. Brave Shields)
  // commonly hide floating corner buttons via attribute selectors like [aria-label="Back to top"].
  // The accessible name comes from a visually-hidden child instead, which those selectors can't match.
  return createPortal(
    <button
      onClick={toTop}
      className={`fixed bottom-5 right-5 z-chrome inline-flex items-center justify-center w-16 h-16 rounded-full glass-card glass-hover text-fg shadow-lg hover:text-accent transition-all duration-200 ${
        show ? "opacity-100 translate-y-0" : "opacity-0 translate-y-3 pointer-events-none"
      }`}
    >
      <ArrowUp className="w-7 h-7" aria-hidden="true" />
      <span className="sr-only">{t("common.backToTop")}</span>
    </button>,
    document.body,
  );
}
