AccumulatedVisualContext Tree:
  [1] scroll_frame_id=1 (PaintableWithLines(BlockContainer<PRE>#out))
    [2] clip=[8,8 200x200] (PaintableWithLines(BlockContainer(anonymous)))
      [4] effects=[opacity=0.5] (PaintableWithLines(BlockContainer<DIV>.abspos))

DisplayList:
SaveLayer@0
  CompositorWheelHitTestTarget@0 target_scroll_frame_index=0 rect=[0,0 800x600]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 800x221]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,8 784x200]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,8 200x200]
  CompositorWheelHitTestTarget@4 target_scroll_frame_index=0 rect=[8,8 100x100]
  FillRect@4 rect=[8,8 100x100] color=rgb(0, 0, 0)
Restore@0