In all iframes, the green fixed-position element should not be composited.
{
  "layers": [
    {
      "name": "Scrolling Contents Layer",
      "bounds": [785, 4016],
      "contentsOpaque": true,
      "backgroundColor": "#FFFFFF"
    },
    {
      "name": "LayoutNGBlockFlow (positioned) DIV class='composited box'",
      "bounds": [300, 100],
      "contentsOpaque": true,
      "backgroundColor": "#00FFFF",
      "transform": 1
    },
    {
      "name": "LayoutIFrame (positioned) IFRAME id='iframe2' class='composited'",
      "bounds": [154, 154],
      "transform": 2
    },
    {
      "name": "LayoutIFrame (positioned) IFRAME id='iframe3'",
      "position": [10, 380],
      "bounds": [154, 154]
    },
    {
      "name": "ContentsLayer for Vertical Scrollbar Layer",
      "position": [785, 0],
      "bounds": [15, 600],
      "contentsOpaque": true
    }
  ],
  "transforms": [
    {
      "id": 1,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [50, 360, 0, 1]
      ]
    },
    {
      "id": 2,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [10, 200, 0, 1]
      ]
    }
  ]
}

Composited box underneath iframe.