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 800x93]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 800x80]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 800x80]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 120x80]
  CompositorMainThreadWheelEventRegion@1 rect=[0,0 120x80]
  Save@1
    AddClipRect@1 rect=[0,0 120x80]
    DrawCompositedContext@1 dst_rect=[0,0 120x80]
  Restore@1
Restore@0