public async Task<IActionResult> OnPostDownloadZipAsync(CancellationToken cancellationToken)
{
    if (!OrgName.TryCreate(User.GetCompanyName(), out var orgName))
        return NotFound();

    var fallback = RedirectToPage(new
    {
        q = Search,
        types = TypeFilterSelection,
        pageSize = PageSize,
        pageToken = PageToken,
    });

    if (SelectedDocuments is null || SelectedDocuments.Count == 0)
        return fallback;

    var relativePaths = SelectedDocuments
        .SelectWhere<string, DocumentRelativePath>(x => new DocumentId(x).TryDecode(out var y) ? y.Value : null)
        .Where(x => IsDownloadable(x.Name))
        .ToList();

    if (relativePaths.Count == 0)
        return fallback;

    var orgSlug = await documents.ResolveCustomerPrefixAsync(OrgId, orgName.Value, cancellationToken);
    if (orgSlug is null)
        return fallback;

    // Zipping compresses each entry with a DeflateStream, which flushes its buffers synchronously
    // when the entry is closed. Kestrel forbids synchronous response writes by default, so we opt
    // in for this streamed response only. Nothing large is written synchronously - the document
    // payloads are still copied with async IO.
    var bodyControl = HttpContext.Features.Get<IHttpBodyControlFeature>();
    bodyControl?.AllowSynchronousIO = true;

    Response.ContentType = "application/zip";
    Response.Headers.ContentDisposition = $"attachment; filename=\"{DocumentsService.ZipDownloadFileName}\"";

    await documents.WriteZipArchiveAsync(orgSlug.Value, relativePaths, Response.Body, cancellationToken);

    return new EmptyResult();
}