Today, CLM::UpdateShouldFlattenTransform calls

  scrolling_layer_->SetShouldFlattenTransform(false);

overriding the default of true.  The comment says having it flatten "causes unclipped render surfaces" and references  issue 521768 .

pdr thinks we might be able to remove this now.  It causes diffs in layer tree output with root layer scrolling, for example in compositing/fixed-body-background-positioned.html, which has:

  "transforms": [
    {
      "id": 1,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [0, -200, 0, 1]
      ],
      "flattenInheritedTransform": false
    }
  ]

Note that the "flattenInheritedTransform" JSON field is present only when false, and is based on the parent GraphicsLayer's should_flatten_transform_ bit.

With RLS, the transform is on the scrolling contents layer, so its parent is the CLM::scrolling_layer_.

Without RLS, the transform is on the document's main GraphicsLayer, so its parent is the PLC::root_content_layer_, which has the default value should_flatten_transform_ == true.