With mask-type alpha:
AccumulatedVisualContext Tree:
  [0] transform=[1,0,0,1,0,0] origin=(0,0) (ViewportPaintable(Viewport<#document>))
    [1] scroll (PaintableWithLines(BlockContainer<PRE>#out))
      [2] clip=[8,8 200x200]
        [3] mask=[-11,-11 238x238] kind=alpha origin=svg-mask (SVGPathPaintable(SVGGeometryBox<rect>))

DisplayList:
SaveLayer@0
  CompositorWheelHitTestTarget@0 target_scroll_node_index=0 rect=[0,0 800x600]
  CompositorWheelHitTestTarget@1 target_scroll_node_index=0 rect=[0,0 800x221]
  CompositorWheelHitTestTarget@1 target_scroll_node_index=0 rect=[8,8 784x200]
  CompositorWheelHitTestTarget@1 target_scroll_node_index=0 rect=[8,8 784x200]
  CompositorWheelHitTestTarget@1 target_scroll_node_index=0 rect=[8,8 200x200]
  FillPath@3 path_bounding_rect=[8,8 100x100]
Restore@0
MaskDisplayList for context 3:
  FillPath@0 path_bounding_rect=[8,8 100x50]
MaskDisplayList for context 4:
  FillPath@0 path_bounding_rect=[8,8 100x50]
MaskDisplayList for context 5:
  FillPath@0 path_bounding_rect=[8,8 100x50]

After switching to mask-type luminance:
AccumulatedVisualContext Tree:
  [0] transform=[1,0,0,1,0,0] origin=(0,0) (ViewportPaintable(Viewport<#document>))
    [1] scroll (PaintableWithLines(BlockContainer<PRE>#out))
      [2] clip=[8,8 200x200]
        [3] mask=[-11,-11 238x238] kind=luminance origin=svg-mask (SVGPathPaintable(SVGGeometryBox<rect>))

DisplayList:
SaveLayer@0
  CompositorWheelHitTestTarget@0 target_scroll_node_index=0 rect=[0,0 800x600]
  CompositorWheelHitTestTarget@1 target_scroll_node_index=0 rect=[0,0 800x221]
  CompositorWheelHitTestTarget@1 target_scroll_node_index=0 rect=[8,8 784x200]
  CompositorWheelHitTestTarget@1 target_scroll_node_index=0 rect=[8,8 784x200]
  CompositorWheelHitTestTarget@1 target_scroll_node_index=0 rect=[8,8 200x200]
  FillPath@3 path_bounding_rect=[8,8 100x100]
Restore@0
MaskDisplayList for context 3:
  FillPath@0 path_bounding_rect=[8,8 100x50]
MaskDisplayList for context 4:
  FillPath@0 path_bounding_rect=[8,8 100x50]
MaskDisplayList for context 5:
  FillPath@0 path_bounding_rect=[8,8 100x50]

