const downloads = new WeakMap<HTMLElement, () => void>();

/** Keep a direct user-gesture fallback until replaced or the editor is removed. */
export function offerBuilderDownload(
  host: HTMLElement,
  content: string,
  type: string,
  filename: string,
): void {
  const outlet = host.querySelector<HTMLElement>("[data-builder-download]");
  if (!outlet) throw new Error("Download outlet is unavailable");
  const url = URL.createObjectURL(new Blob([content], { type }));
  const link = host.ownerDocument.createElement("a");
  link.href = url;
  link.download = filename;
  link.textContent = `Download prepared file: ${filename}`;
  let disposed = false;
  const observer = new MutationObserver(() => {
    if (!host.isConnected) dispose();
  });
  const dispose = () => {
    if (disposed) return;
    disposed = true;
    observer.disconnect();
    URL.revokeObjectURL(url);
    link.remove();
    if (downloads.get(host) === dispose) downloads.delete(host);
  };
  try {
    downloads.get(host)?.();
    outlet.replaceChildren(link);
    observer.observe(host.ownerDocument.documentElement, { childList: true, subtree: true });
    downloads.set(host, dispose);
  } catch (error) {
    dispose();
    throw error;
  }
  // A blocked automatic click must not destroy the direct-click fallback.
  try {
    link.click();
  } catch {
    /* The visible link remains available. */
  }
}