root concept: order of use and overwriting

[muzimuzhi]

Brief outline of the bug

There are two problems with mindmap style root concept:

• Style mindmap uses root concept, hence should be used before any modification to root concept.
  This is reported by @claell in Color transitions of mindmap nodes are not always applied automatically #1165 (comment).
• Even worse, derived styles like small mindmap redefine root concept (using handler /.style) then use mindmap. Thus user modification to root concept never takes effect.

Changing root concept/.style to root concept/.append style in definitions of styles like small mindmap will solve the problem, though not very elegant. And uses of wrongly ordered mindmap, ..., root concept/.append style=... in pgfmanual, last example for key /tikz/concept color should be adjusted.

In my understanding, root concept is used by style mindmap rather than specified by user as in \node[root concept] {...} child {...}; because it's currently impossible to detect level 0 root node for node tree only. This would be enabled when node parser is changed to first parse and collect node components. This way it will know whether the first node is a single node or the root node of a node tree.

Minimal working example (MWE)

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{mindmap}

\begin{document}
  % Modifying root concept` before `mindmap` works
  \tikz[root concept/.append style={concept color=blue!80},
        mindmap]
    \node [concept] {Root concept}
      child[concept color=red,grow=0] {node[concept] {Child concept}};

  % Modifying `root concept` after `mindmap` has no effect
  \tikz[mindmap,
        root concept/.append style={concept color=blue!80}]
    \node [concept] {Root concept}
      child[concept color=red,grow=0] {node[concept] {Child concept}};

  % Modifying `root concept` has no effect on styles like `small mindmap`.
  \tikz[small mindmap,
        root concept/.append style={concept color=blue!80}]
    \node [concept] {Root concept}
      child[concept color=red,grow=0] {node[concept] {Child concept}};
  \qquad
  \tikz[root concept/.append style={concept color=blue!80},
        small mindmap]
    \node [concept] {Root concept}
      child[concept color=red,grow=0] {node[concept] {Child concept}};
\end{document}