This scenario verifies that the cyan "container" element scrolls properly with squashing enabled. The "container" element should not squash into a composited layer mapping owned by the fixed position layer or its descendant, since this would make it behave like a fixed position element during composited scrolling.

CASE 1, original layer tree:
{
  "layers": [
    {
      "name": "Scrolling background of LayoutView #document",
      "bounds": [785, 4050],
      "contentsOpaque": true,
      "backgroundColor": "#FFFFFF"
    },
    {
      "name": "LayoutNGBlockFlow (positioned) DIV id='fixed'",
      "bounds": [400, 200],
      "contentsOpaque": true,
      "backfaceVisibility": "hidden",
      "backgroundColor": "#0000FF"
    },
    {
      "name": "LayoutNGBlockFlow (positioned) DIV id='container'",
      "position": [100, 50],
      "bounds": [200, 4000],
      "contentsOpaque": true,
      "backgroundColor": "#00FFFF"
    },
    {
      "name": "VerticalScrollbar",
      "position": [785, 0],
      "bounds": [15, 600],
      "contentsOpaque": true
    }
  ]
}

CASE 2, scrolling y to 80, the "container" element should remain positioned with respect to the scrolled document, the fixed-pos layer compensates for the new scroll position:
{
  "layers": [
    {
      "name": "Scrolling background of LayoutView #document",
      "bounds": [785, 4050],
      "contentsOpaque": true,
      "backgroundColor": "#FFFFFF",
      "transform": 1
    },
    {
      "name": "LayoutNGBlockFlow (positioned) DIV id='fixed'",
      "bounds": [400, 200],
      "contentsOpaque": true,
      "backfaceVisibility": "hidden",
      "backgroundColor": "#0000FF"
    },
    {
      "name": "LayoutNGBlockFlow (positioned) DIV id='container'",
      "position": [100, 50],
      "bounds": [200, 4000],
      "contentsOpaque": true,
      "backgroundColor": "#00FFFF",
      "transform": 1
    },
    {
      "name": "VerticalScrollbar",
      "position": [785, 0],
      "bounds": [15, 600],
      "contentsOpaque": true
    }
  ],
  "transforms": [
    {
      "id": 1,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [0, -80, 0, 1]
      ]
    }
  ]
}