In all iframes, the green fixed-position element should not be composited.
{
  "bounds": [785, 4016],
  "children": [
    {
      "bounds": [785, 4016],
      "contentsOpaque": true,
      "drawsContent": true,
      "children": [
        {
          "position": [50, 360],
          "bounds": [300, 100],
          "contentsOpaque": true,
          "drawsContent": true,
          "backgroundColor": "#00FFFF"
        },
        {
          "position": [10, 200],
          "bounds": [154, 154],
          "drawsContent": true
        },
        {
          "position": [10, 380],
          "bounds": [154, 154],
          "drawsContent": true
        }
      ]
    }
  ]
}
Composited box underneath iframe.