Draw stroke inside/outside of path for arbitrary shapes and nodes

Question

This is a followup question of Is there a way to draw TikZ lines on the “inside” or “outside” of a path?

Since the other question is quite old I want to ask, if TikZ in the meantime is able to draw a stroke inside, on or outside of a path, like in this picture?

This was generated with the following code using \pgflinewidth to change the actual path. But this solution doesn’t work for arbitrary shapes and not for nodes, furthermore it is broken when scaling the picture.

\documentclass[border=5mm,tikz]{standalone}

\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}[
   every node/.style = {below=5mm, text=black, font=\small},
%   scale = 1.5,
]
   % path
   \draw [red] (0,0) rectangle +(2,2)
      +(1,0) node {actual path};
   % stroke on path
   \draw [line width = 4mm] (3,0) rectangle +(2,2);
   \draw [red] (3,0) rectangle +(2,2)
      +(1,0) node {stroke on path};
   % stroke inside path
   \draw [line width = 4mm]
      ($(6,0)+(\pgflinewidth/2,\pgflinewidth/2)$) rectangle
      +($(2,2)-(\pgflinewidth,\pgflinewidth)$);
   \draw [red] (6,0) rectangle +(2,2)
      +(1,0) node {stroke inside path};
   % stroke outsie path
   \draw [line width = 4mm]
      ($(9,0)-(\pgflinewidth/2,\pgflinewidth/2)$) rectangle
      +($(2,2)+(\pgflinewidth,\pgflinewidth)$);
   \draw [red] (9,0) rectangle +(2,2)
      +(1,0) node {stroke outside path};
\end{tikzpicture}
\end{document}

Actually I need it for drawing a row of nodes where some of them are filled and some are drawn, but the total hight should be equal and they should’t overlap. This is how it should not look like:

Instead the stroke of “State B” and “State C” should lie inside of the path. (The postaction is only for demonstration.)

Code:

\begin{tikzpicture}[
   every node/.style = {
      inner sep=0pt, outer sep=0pt, postaction = {draw, red, thin},
      minimum height = 8mm, anchor = south west, font=\small
   },
   a/.style = {fill = gray},
   b/.style = {draw, line width = 2mm},
   c/.style = {draw, line width = 2mm, shape = signal },
]
   \node at (0,0) [minimum width = 20mm, a] {State A};
   \node at (2,0) [minimum width = 20mm, b] {State B};
   \node at (4,0) [minimum width = 30mm, a] {State A};
   \node at (7,0) [minimum width = 30mm, c] {State C};
   \draw [->] (0,-0.5) -- +(10.5,0);
   \foreach \x in {0,0.5,...,10}
      \draw (\x,-0.6) -- (\x,-0.4);
\end{tikzpicture}

Answer 1

It is somewhat easy to come up with a decoration which draws the outside of a path. In principle, one could reverse the path to get the curve drawn inside but that has some unforeseen consequences.

\documentclass{article}

\usepackage{tikz}    
\usetikzlibrary{calc,decorations}

\pgfdeclaredecoration{stroke outside path}{initial}{%
  \state{initial}[width=\pgfdecoratedinputsegmentlength/100,next state=iterate]
  {
    \pgfpathmoveto{\pgfqpoint{0pt}{.5\pgflinewidth}}
  }%
  \state{iterate}[width=\pgfdecoratedinputsegmentlength/100]
  {
    \pgfpathlineto{\pgfqpoint{.5\pgflinewidth}{.5\pgflinewidth}}
  }%
  \state{final}{%
    \pgfpathlineto{\pgfpointadd{\pgfqpoint{\pgflinewidth}{.5\pgflinewidth}}{\pgfpointdecoratedpathlast}}
  }%
}%

\begin{document}
\begin{tikzpicture}[
   every node/.style = {below=5mm, text=black, font=\small},
%   scale = 1.5,
]
   % path
   \draw [red] (0,0) rectangle +(2,2)
      +(1,0) node {actual path};
   % stroke on path
   \draw [line width = 4mm] (3,0) rectangle +(2,2);
   \draw [red] (3,0) rectangle +(2,2)
      +(1,0) node {stroke on path};
   % stroke inside path
   \draw [decoration={reverse path, stroke outside path}, decorate, line width = 4mm] (6,0) rectangle +(2,2);
   \draw [red] (6,0) rectangle +(2,2)
      +(1,0) node {stroke inside path};
   % stroke outsie path
   \draw [decoration=stroke outside path, decorate, line width = 4mm] (9,0) rectangle +(2,2);
   \draw [red] (9,0) rectangle +(2,2)
      +(1,0) node {stroke outside path};
\end{tikzpicture}

\begin{tikzpicture}
  \draw [decoration=stroke outside path, decorate, line width = 4mm] (0,0) .. controls (1,-1) and (2,2) .. (3,0);
  \draw [red] (0,0) .. controls (1,-1) and (2,2) .. (3,0);
\end{tikzpicture}
\end{document}

Answer 2

For this solution you need to load the calc library. Then we will basically draw a line for the b style, appended after the node has been set. Without calc, the line would appear like in your example, but using it, we can add or remove the \pgflinewidth.

I haven't tested it, but it should be scalable.

Output

Code

\documentclass[margin=10pt]{standalone}
\usepackage{pgffor}
\usepackage{subcaption}
\usepackage{tikz}

\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}[
    every node/.style = {
       inner sep=0pt, outer sep=0pt, postaction = {draw, red, thin},
       minimum height = 8mm, anchor = south west, font=\small
    },
    a/.style = {fill = gray},
    b/.style = {append after command={\pgfextra{
        \draw[line width=1mm] ($(\tikzlastnode.south west)+(.5\pgflinewidth,.5\pgflinewidth)$) rectangle ($(\tikzlastnode.north east)+(-.5\pgflinewidth,-.5\pgflinewidth)$);
    }}},
]
   \node at (0,0) [minimum width = 20mm, a] {State A};
   \node at (2,0) [minimum width = 20mm, b] {State B};
   \node at (4,0) [minimum width = 30mm, a] {State A};
\end{tikzpicture}
\end{document}