code: function lockFirstScreen () {
    // when is prerendering, iframe container display none,
    // all elements are not in viewport.
    if (prerender.isPrerendering) {
      return
    }
    let viewportRect = viewport.getRect()
    fsElements = fsElements.filter((element) => {
      if (prerender.isPrerendered) {
        return element._resources.isInViewport(element, viewportRect)
      }
      return element.inViewport()
    }).map((element) => {
      element.setAttribute('mip-firstscreen-element', '')
      return element
    })
    fsElementsLocked = true
    tryRecordFirstScreen()
    !prerender.isPrerendered && firstScreenLabel.sendLog()
  }
docs: Lock the fsElements. No longer add fsElements.