"append after command" in a tree (child path operation)

Question

Related: Problem with "append after command" and "insert path", but I did not understand how to apply the solutions there in my case.

I'm defining a simple tikz style which uses append after command:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning,fit}
\begin{document}
\begin{tikzpicture}[
  fit after/.style={
    append after command={%
      node[draw, inner sep=0pt, fit=(\tikzlastnode)] {}
    }
  }
  ]
  \node[fit after] {A};%%%% WORKS
  \node {B} child { node[fit after] {C} }; %%%% FAILS
\end{tikzpicture}
\end{document}

On the first node (A), it works fine and draws a rectangle around it.

On the C node, however, I get the following error message:

Package pgf Error: No shape named  is known.

The \tikzlastnode isn't at fault, as even hardcoding the name of the node still fails in the child case.

Is the child path operation messing with append after command? Is there a workaround?

Context: I am trying to make a triangle fit whole subtree command, which draws a triangle that fits around all the nodes in the subtree.

Answer 1

If forest is an option, it is reasonably straightforward to define a fit whole subtree style. I don't recommend a triangular shape. Although I experimented a bit, this inevitably looked very ugly. However, fit whole subtree can be passed whatever options you wish to style the node. By default, it draws a rectangle in red.

Note that multiple uses of fit whole subtree, including nested uses, work fine.

\documentclass[tikz,border=10pt,multi]{standalone}
\usepackage{forest}
\usetikzlibrary{fit}
\begin{document}
\newcommand*\makenodename[1]{(#1)}
\forestset{
  declare toks={fit these}{},
  fit whole subtree/.style={
    delay={
      temptoksa=,
      for tree={%
        temptoksa+/.wrap pgfmath arg={ (##1)}{name()},
      },
      fit these/.register=temptoksa,
      delay={
        tikz+={
          \node [draw=red, fit=\foresteoption{fit these}, inner sep=0pt, #1] {};
        },
      },
    },
  },
}
\begin{forest}
  for tree={
    fit=band,
    child anchor=parent,
    parent anchor=children,
  }
  [A
    [B
      [C, fit whole subtree
        [D]
        [E
          [X]
          [Y
            [Z]
          ]
        ]
        [F]
        [G]
      ]
      [H
        [I
          [J]
          [K]
          [L]
        ]
      ]
    ]
    [M, fit whole subtree={thick, draw=blue, rounded corners}
      [N
        [O
          [R]
        ]
        [P]
        [Q]
      ]
      [S, fit whole subtree={green, densely dashed, rounded corners}
        [T]
        [U]
        [V]
        [W]
      ]
    ]
  ]
\end{forest}
\end{document}