Adding Tikz node in pgfextra triggers double drawing of containing path

Question

This is a follow up question to Adding nodes in a style

If I use the following code, I obtain a strange behavior:

Code

\documentclass[tikz]{standalone}
\usetikzlibrary{matrix,fit,backgrounds}
\tikzset{
  fill first cell/.style={
    append after command={
      \pgfextra
        \pgfonlayer{background}
          \tikzset{every node/.style=}
          \node[fit=(\tikzlastnode-\the\pgfmatrixcurrentcolumn-\the\pgfmatrixcurrentrow), inner sep=+0pt, fill=#1] {};
        \endpgfonlayer
      \endpgfextra
    }
  },
  fill first cell/.default=red!7
}
\begin{document}
\begin{tikzpicture}
\matrix (m) [matrix of nodes, fill first cell] {
  1 & 2 \\
  3 & 4 \\
};
\end{tikzpicture}
\end{document}

Output

Desired output

Everything looks as expected when printed. However, the PDF viewer shows all the letters slightly bolder. This behavior can be seen in the two outputs shown above. I strongly suspect that the content of the matrix is drawn twice.

I know it is drawn at least once after the code from my style. To prove it, remove the background layer command and the red rectangle will still appear in the background from the number 4.

Furthermore, if I use a pgfnode instead of a Tikz node the problem does not appear.

What is causing this behavior and how can I fix it?

Edit

With the help of @percusse, @DavidCarlisle, and @AndrewSwann I have been able to find a shorter MWE as well as a better way to check for the existence of the bug.

The following code exhibits the bug:

\documentclass[tikz]{standalone}
\begin{document}
\showoutput
\begin{tikzpicture}
\path node {x}
      \pgfextra
        \node{};
      \endpgfextra
;
\end{tikzpicture}
\end{document}

By using the following command pdflatex mwe.tex | grep cmr/ | wc -l you can check how many times the character x is being printed. 1 means no bug, 2 means bug.

By playing with the content of \pgfextra ... \endpgfextra I have made some additional observations:

• \pgfnode{rectangle}{node}{}{}{} does not exhibit the bug, so it seems to be a Tikz problem
• \draw (0,0) -- (0,1) -- (1,1) -- (1,0) -- cycle; does not exhibit the bug either
• \draw (0,0) -- (0,1) -- (1,1) -- (1,0) -- cycle node {}; does exhibit the bug
• \draw (0,0) -- (0,1) -- (1,1) -- (1,0) -- cycle; \node {}; does not exhibit the bug

The lines composing the path do not seem to be drawn twice. However, in the following code, both nodes, x and y, are printed twice.

\documentclass[tikz]{standalone}
\begin{document}
\showoutput
\begin{tikzpicture}
\path node {x} node {y}
      \pgfextra
        \node{};
      \endpgfextra
;
\end{tikzpicture}
\end{document}

Workaround (Obsolete, see the solution instead)

This question is not yet answered, however there is now a workaround available: just add a \path; before the \node{}; inside \pgfextra.

Solution

As mentionned in Adding nodes in a style and in @MarkWibrow's answer, the problem was a lack of protection of the outer path in \pgfextra. The classical way to protect is to use \pgfinterruptpath. This is however not enough when inserting TikZ nodes and it must be coupled with \setbox\tikz@figbox=\box\pgfutil@voidb@x. The fixed MWE looks like:

\documentclass[tikz]{standalone}
\begin{document}
\showoutput
\begin{tikzpicture}
\path node {x}
      \pgfextra
        \pgfinterruptpath
          \makeatletter\setbox\tikz@figbox=\box\pgfutil@voidb@x\makeatother
          \node{};
        \endpgfinterruptpath
      \endpgfextra
;
\end{tikzpicture}
\end{document}

The \pgfinterrupath serves two purposes. If on top of a node x we were drawing a path, it would make sure the node y would not delete it. In our case, it is needed for \makeatletter\setbox\tikz@figbox=\box\pgfutil@voidb@x\makeatother not discard the x node.

This solution works for TikZ/PGF version 2.10 and the CVS version. The problem mentionned by @MarkWibrow in the CVS version was the following: When the fit node was encountered, the nodes of the matrix were all inserted in the background layer instead of the main layer where they belong. This was a bug (http://sourceforge.net/p/pgf/bugs/260/) that is now fixed. If using exclusively the CVS version (after September 9 2013) the following code works:

\documentclass[tikz]{standalone}
\begin{document}
\showoutput
\begin{tikzpicture}
\path node {x}
      \pgfextra
        \pgfinterruptpath
          \node{};
        \endpgfinterruptpath
      \endpgfextra
;
\end{tikzpicture}
\end{document}

Answer 1

\documentclass[tikz]{standalone}
\usetikzlibrary{matrix,fit,backgrounds}
\tikzset{
  fill first cell/.style={
    append after command={
      \pgfextra
        \pgfonlayer{background}
          \tikzset{every node/.style=}
          \node[fit=(\tikzlastnode-\the\pgfmatrixcurrentcolumn-\the\pgfmatrixcurrentrow), inner sep=+0pt, fill=#1] {};
        \endpgfonlayer
      \endpgfextra
    }
  },
  fill first cell/.default=red!7
}
\begin{document}
\showoutput
\begin{tikzpicture}
\matrix (m) [matrix of nodes, fill first cell] {
  1 & 2 \\
  3 & 4 \\
};
\end{tikzpicture}
\end{document}

Then look at the log

$ grep "cmr/m/n/10" tkmatrix.log
.........................\OT1/cmr/m/n/10 1
.........................\OT1/cmr/m/n/10 2
.........................\OT1/cmr/m/n/10 3
.........................\OT1/cmr/m/n/10 4
.........................\OT1/cmr/m/n/10 1
.........................\OT1/cmr/m/n/10 2
.........................\OT1/cmr/m/n/10 3
.........................\OT1/cmr/m/n/10 4

You are correct in your observation that the matrix is drawn twice.

Answer 2

The matrix is being drawn twice and this is caused by the presence of a drawing command in your \pgfextra. Here is a test file to show the behaviour.

\documentclass[tikz]{standalone}

\pdfcompresslevel=0

\tikzset{ff/.style={append after command={\pgfextra\node {};\endpgfextra}}}

\begin{document}
\begin{tikzpicture}
\node[ff] (0,0) {XY};
\end{tikzpicture}
\end{document}

pgfcompresslevel=0 means that the resulting pdf file is in more readable form. From the above code you get the following pdf code twice in the output

BT
/F8 9.9626 Tf 3.321 3.321 Td [(XY)]TJ
ET

the first on lines 26-28, the second on lines 51-53. Removing \node {}; from the pdfextra material, produces a pdf file with only one occurance of the printing command.

Note that I have not used any backgroud, fit or matrix libraries for this.

Answer 3

All nodes in a path are positioned in a special box called \tikz@figbox. When something is done with nodes, something like the following occurs at some point:

\setbox\tikz@figbox=\hbox\bgroup%
   \setbox\pgfutil@tempboxa=\copy\tikz@figbox%
   \unhbox\pgfutil@tempboxa%
   %
   % Code inserting/positioning node content
   %
\egroup%

At the end of a path the box is used and becomes void (which is why the insertion of \path; seems to work).

In the case given above the OP starts a path (a \node is a special kind of path - the \path is inserted automatically) inside another path (a \matrix is a type of node, which is, therefore, a kind of path).

As the \tikz@figbox contents from the matrix are not voided before the \node command, the matrix contents are also inserted into the box for the node contents. It is possible to put \setbox\tikz@figbox=\box\pgfutil@voidb@x before the node command to see that this corrects the issue.

Effectively what the OP has done (more or less) is this:

\newbox\mybox
\setbox\mybox=\hbox{matrix}
{\setbox\mybox=\hbox{\copy\mybox\space node}1. \box\mybox}
2. \box\mybox

Which produces

1. matrix node 2. matrix

I personally don't see it as a bug, the approach given by the OP is not really a good way to do things. \pgfextra has to be used with care if other paths/pictures are being used (e.g., using \pgfinuterruptpath \endpgfinterruptpath). Although in this case it doesn't solve the problem.

Either way, the duplication does not occur with the latest CVS versions, but it is also the case that the append after command key doesn't produce the required effect either.