Before background-attachment change:
AccumulatedVisualContext Tree:
  [0] transform=[1,0,0,1,0,0] origin=(0,0) (ViewportPaintable(Viewport<#document>))
    [1] scroll_frame_id=1 (PaintableWithLines(BlockContainer<PRE>#out))

DisplayList:
SaveLayer@0
  CompositorWheelHitTestTarget@0 target_scroll_frame_index=0 rect=[0,0 800x600]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 800x213]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 800x200]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 200x200]
  Save@1
    AddClipRect@1 rect=[0,0 200x200]
    PaintLinearGradient@1 rect=[0,0 200x600]
  Restore@1
Restore@0

After background-attachment change:
AccumulatedVisualContext Tree:
  [0] transform=[1,0,0,1,0,0] origin=(0,0) (ViewportPaintable(Viewport<#document>))
    [1] scroll_frame_id=1 (PaintableWithLines(BlockContainer<PRE>#out))
      [2] scroll_compensation(frame_id=1)

DisplayList:
SaveLayer@0
  CompositorWheelHitTestTarget@0 target_scroll_frame_index=0 rect=[0,0 800x600]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 800x213]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 800x200]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 200x200]
  Save@1
    AddClipRect@1 rect=[0,0 200x200]
    PaintLinearGradient@2 rect=[0,0 800x600]
  Restore@1
Restore@0