Even though the fixed-position element's container is nonscrollable, it should still be composited because one of its ancestors is scrolling.

{
  "layers": [
    {
      "name": "Scrolling background of LayoutView #document",
      "bounds": [800, 600],
      "contentsOpaque": true,
      "backgroundColor": "#FFFFFF"
    },
    {
      "name": "LayoutBlockFlow DIV class='scrollable bigBox'",
      "position": [1, 1],
      "bounds": [300, 300],
      "drawsContent": false,
      "transform": 1
    },
    {
      "name": "LayoutBlockFlow DIV class='scrollable bigBox'",
      "position": [1, 1],
      "bounds": [300, 800],
      "transform": 1
    },
    {
      "name": "VerticalScrollbar",
      "position": [297, 1],
      "bounds": [4, 300],
      "transform": 1
    },
    {
      "name": "LayoutBlockFlow (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],
        [8, 68, 0, 1]
      ]
    },
    {
      "id": 2,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [10, 100, 0, 1]
      ]
    }
  ]
}