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+0 120 0+0+0] [BFC] children: not-inline
    BlockContainer <body> at [8,8] [8+0+0 784 0+0+8] [8+0+0 104 0+0+8] children: inline
      frag 0 from BlockContainer start: 0, length: 0, rect: [13,10 143.515625x100] baseline: 56.796875
      BlockContainer <button> at [13,10] inline-block [0+1+4 143.515625 4+1+0] [0+1+1 100 1+1+0] [BFC] children: not-inline
        BlockContainer <(anonymous)> at [13,10] flex-container(column) [0+0+0 143.515625 0+0+0] [0+0+0 100 0+0+0] [FFC] children: not-inline
          BlockContainer <(anonymous)> at [13,51] flex-item [0+0+0 143.515625 0+0+0] [0+0+0 18 0+0+0] [BFC] children: inline
            frag 0 from TextNode start: 0, length: 19, rect: [13,51 143.515625x18] baseline: 13.796875
                "Middle-aligned text"
            TextNode <#text> (not painted)
      TextNode <#text> (not painted)

ViewportPaintable (Viewport<#document>) [0,0 800x600]
  PaintableWithLines (BlockContainer<HTML>) [0,0 800x120]
    PaintableWithLines (BlockContainer<BODY>) [8,8 784x104]
      PaintableWithLines (BlockContainer<BUTTON>) [8,8 153.515625x104]
        PaintableWithLines (BlockContainer(anonymous)) [13,10 143.515625x100]
          PaintableWithLines (BlockContainer(anonymous)) [13,51 143.515625x18]
            TextPaintable (TextNode<#text>)

SC for Viewport<#document> [0,0 800x600] [children: 1] (z-index: auto)
 SC for BlockContainer<HTML> [0,0 800x120] [children: 0] (z-index: auto)
