When the parent document becomes composited, the layer trees should get connected together.

{
  "layers": [
    {
      "name": "Scrolling background of LayoutView #document",
      "bounds": [800, 600],
      "contentsOpaque": true,
      "backgroundColor": "#FFFFFF"
    },
    {
      "name": "Scrolling background of LayoutView #document",
      "bounds": [285, 230],
      "contentsOpaque": true,
      "backgroundColor": "#C0C0C0",
      "transform": 1
    },
    {
      "name": "LayoutNGBlockFlow DIV id='iframe-content' class='box'",
      "bounds": [210, 210],
      "contentsOpaque": true,
      "backgroundColor": "#0000FF",
      "transform": 2
    },
    {
      "name": "VerticalScrollbar",
      "position": [285, 0],
      "bounds": [15, 150],
      "contentsOpaque": true,
      "transform": 1
    },
    {
      "name": "LayoutNGBlockFlow DIV id='box' class='composited'",
      "bounds": [100, 100],
      "contentsOpaque": true,
      "backgroundColor": "#0000FF",
      "transform": 3
    }
  ],
  "transforms": [
    {
      "id": 1,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [43, 143, 0, 1]
      ]
    },
    {
      "id": 2,
      "parent": 1,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [18, 10, 0, 1]
      ]
    },
    {
      "id": 3,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [8, 8, 0, 1]
      ]
    }
  ]
}