Before:
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 800x85]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,8 784x64]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,8 100x64]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,8 69.625x16]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,24 47.0625x16]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,40 60x16]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,56 36.46875x16]
  DrawGlyphRun@1 rect=[8,8 70x16] translation=[8,20.796875] color=rgb(0, 0, 255)
  DrawLine@1 from=[8,24] to=[78,24] color=rgb(0, 0, 255) thickness=2
  DrawGlyphRun@1 rect=[8,24 48x16] translation=[8,36.796875] color=rgb(0, 0, 255)
  DrawLine@1 from=[8,40] to=[55,40] color=rgb(0, 0, 255) thickness=2
  DrawGlyphRun@1 rect=[8,40 60x16] translation=[8,52.796875] color=rgb(0, 0, 255)
  DrawLine@1 from=[8,56] to=[68,56] color=rgb(0, 0, 255) thickness=2
  DrawGlyphRun@1 rect=[8,56 37x16] translation=[8,68.796875] color=rgb(0, 0, 255)
  DrawLine@1 from=[8,72] to=[44,72] color=rgb(0, 0, 255) thickness=2
Restore@0

After:
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 800x85]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,8 784x64]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,8 100x64]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,8 69.625x16]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,24 47.0625x16]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,40 60x16]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,56 36.46875x16]
  DrawGlyphRun@1 rect=[8,8 70x16] translation=[8,20.796875] color=rgb(255, 0, 0)
  DrawLine@1 from=[8,24] to=[78,24] color=rgb(255, 0, 0) thickness=2
  DrawGlyphRun@1 rect=[8,24 48x16] translation=[8,36.796875] color=rgb(255, 0, 0)
  DrawLine@1 from=[8,40] to=[55,40] color=rgb(255, 0, 0) thickness=2
  DrawGlyphRun@1 rect=[8,40 60x16] translation=[8,52.796875] color=rgb(255, 0, 0)
  DrawLine@1 from=[8,56] to=[68,56] color=rgb(255, 0, 0) thickness=2
  DrawGlyphRun@1 rect=[8,56 37x16] translation=[8,68.796875] color=rgb(255, 0, 0)
  DrawLine@1 from=[8,72] to=[44,72] color=rgb(255, 0, 0) thickness=2
Restore@0