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

{
  "bounds": [800, 600],
  "children": [
    {
      "bounds": [800, 600],
      "contentsOpaque": true,
      "drawsContent": true,
      "children": [
        {
          "position": [8, 68],
          "bounds": [302, 302],
          "shouldFlattenTransform": false,
          "drawsContent": true,
          "children": [
            {
              "position": [1, 1],
              "bounds": [285, 285],
              "shouldFlattenTransform": false,
              "children": [
                {
                  "bounds": [285, 800],
                  "drawsContent": true
                }
              ]
            },
            {
              "bounds": [302, 302],
              "children": [
                {
                  "position": [1, 286],
                  "bounds": [285, 15]
                },
                {
                  "position": [286, 1],
                  "bounds": [15, 285]
                },
                {
                  "position": [286, 286],
                  "bounds": [15, 15],
                  "drawsContent": true
                }
              ]
            }
          ]
        },
        {
          "position": [10, 100],
          "bounds": [100, 100],
          "contentsOpaque": true,
          "drawsContent": true,
          "backgroundColor": "#00FF00"
        }
      ]
    }
  ]
}