next.js/packages/next/src/lib/metadata/generate/icon-mark.tsx

'use client'

// This is a client component that only renders during SSR,
// but will be replaced during streaming with an icon insertion script tag.
// We don't want it to be presented anywhere so it's only visible during streaming,
// right after the icon meta tags so that browser can pick it up as soon as it's rendered.
// Note: we don't just emit the script here because we only need the script if it's not in the head,
// and we need it to be hoistable alongside the other metadata but sync scripts are not hoistable.
export const IconMark = () => {
  if (typeof window !== 'undefined') {
    return null
  }
  return <meta name="«nxt-icon»" />
}