In all iframes, the green fixed-position element should not be composited.
(GraphicsLayer
  (bounds 785.00 4016.00)
  (children 1
    (GraphicsLayer
      (bounds 785.00 4016.00)
      (contentsOpaque 1)
      (drawsContent 1)
      (children 3
        (GraphicsLayer
          (position 50.00 360.00)
          (bounds 300.00 100.00)
          (contentsOpaque 1)
          (drawsContent 1)
          (backgroundColor #00FFFF)
        )
        (GraphicsLayer
          (position 10.00 200.00)
          (bounds 154.00 154.00)
          (drawsContent 1)
        )
        (GraphicsLayer
          (position 10.00 380.00)
          (bounds 154.00 154.00)
          (drawsContent 1)
        )
      )
    )
  )
)
Composited box underneath iframe.