Viewport <#document> at [0,0] [0+0+0 800 0+0+0] [0+0+0 600 0+0+0] children: not-inline
  BlockContainer <html> at [0,0] [0+0+0 800 0+0+0] [0+0+0 600 0+0+0] [BFC] children: not-inline
    BlockContainer <body> at [0,0] [0+0+0 800 0+0+0] [0+0+0 600 0+0+0] children: not-inline
      Box <div#Main> at [0,0] [0+0+0 800 0+0+0] [0+0+0 600 0+0+0] [GFC] children: not-inline
        Box <div#MiddleColumn> at [0,0] flex-container(row) [0+0+0 800 0+0+0] [0+0+0 600 0+0+0] [FFC] children: not-inline
          BlockContainer <div.MiddleHeader> at [0,0] flex-item [0+0+0 100 0+0+0] [0+0+0 100 0+0+0] [BFC] children: not-inline

ViewportPaintable (Viewport<#document>) [0,0 800x600]
  PaintableWithLines (BlockContainer<HTML>) [0,0 800x600]
    PaintableWithLines (BlockContainer<BODY>) [0,0 800x600]
      PaintableBox (Box<DIV>#Main) [0,0 800x600]
        PaintableBox (Box<DIV>#MiddleColumn) [0,0 800x600]
          PaintableWithLines (BlockContainer<DIV>.MiddleHeader) [0,0 100x100]

SC for Viewport<#document> [0,0 800x600] [children: 1] (z-index: auto)
 SC for BlockContainer<HTML> [0,0 800x600] [children: 0] (z-index: auto)
