Even though we can opt-out of fixed-position compositing for unscrollable fixed-position containers, we still need to composite fixed-position layers that need compositing for other reasons such as overlap.

{
  "layers": [
    {
      "name": "Scrolling background of LayoutView #document",
      "bounds": [800, 4024],
      "contentsOpaque": true,
      "backgroundColor": "#FFFFFF"
    },
    {
      "name": "LayoutNGBlockFlow (positioned) DIV class='absolute composited red box'",
      "bounds": [100, 100],
      "contentsOpaque": true,
      "backgroundColor": "#FF0000",
      "transform": 1
    },
    {
      "name": "LayoutNGBlockFlow (positioned) DIV class='fixed lime box'",
      "bounds": [100, 100],
      "contentsOpaque": true,
      "backgroundColor": "#00FF00",
      "transform": 2
    }
  ],
  "transforms": [
    {
      "id": 1,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [10, 100, 0, 1]
      ]
    },
    {
      "id": 2,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [10, 100, 0, 1]
      ]
    }
  ]
}