Before clip 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 800x21]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[8,13 100x100]
  FillRect@1 rect=[8,13 100x100] color=rgb(0, 128, 0)
Restore@0

After clip 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] clip=[8,13 50x50] (PaintableWithLines(BlockContainer<DIV>#box))

DisplayList:
SaveLayer@0
  CompositorWheelHitTestTarget@0 target_scroll_frame_index=0 rect=[0,0 800x600]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=0 rect=[0,0 800x21]
  CompositorWheelHitTestTarget@2 target_scroll_frame_index=0 rect=[8,13 100x100]
  FillRect@2 rect=[8,13 100x100] color=rgb(0, 128, 0)
Restore@0