How can you properly build a genuinely fully custom layout widget by directly extending MultiChildLayoutDelegate or CustomMultiChildLayout, for a genuinely specific complex arrangement not already covered by any of Flutter's own existing standard layout widgets?

For a genuinely specific complex arrangement not already covered directly by combining Flutter's own existing standard layout widgets like Row, Column, and Stack together, you can build a genuinely fully custom layout using CustomMultiChildLayout together with your own custom MultiChildLayoutDelegate, giving you truly complete, full control over precisely positioning and sizing each individual one of several separate distinctly named children based on your own genuinely fully custom specific layout logic, though this particular approach does genuinely require a considerably deeper understanding of Flutter's own underlying low level layout protocol compared to simply composing together its own already existing higher level standard widgets instead.

class MyLayoutDelegate extends MultiChildLayoutDelegate {
  @override
  void performLayout(Size size) {
    positionChild('header', Offset(0, 0));
    positionChild('body', Offset(0, 60));
  }
}