latex nicematrix -- access generated nodes from tikz

Question

Is it normal that I cannot access the by nicematrix generated Tikz nodes within the same align environment?

Example:

\begin{align*}
    \begin{pNiceMatrix}[name=myMatrix]
        1 & 1 & 1 \\
        2 & 2 & 2 \\
    \end{pNiceMatrix}
    \begin{tikzpicture}[remember picture, overlay]
        \draw (myMatrix-1-1) -- (myMatrix-1-2);
    \end{tikzpicture}
\end{align*}

won't work, but

\begin{align*}
    \begin{pNiceMatrix}[name=myMatrix]
        1 & 1 & 1 \\
        2 & 2 & 2 \\
    \end{pNiceMatrix}
\end{align*}
\begin{tikzpicture}[remember picture, overlay]
    \draw (myMatrix-1-1) -- (myMatrix-1-2);
\end{tikzpicture}

works.

This is a bit inconvenient when one wants to stay in the same align environment after the pNiceMatrix (otherwise there will be free space between the matrix and the next line in the same environment and writing in the same line as the matrix is not possible (at least it's difficult when writing macros).

Is there a way to "fix" this?

Answer 1

The environment {align} of amsmath compiles its contents twice. The aim of the first compilation is to measure the width of the content of the environment.

The package nicematrix does not create the PGF/Tikz nodes during that first compilation.

However, you can write a command which will execute its argument only during the second compilation:

\documentclass{article}

\usepackage{nicematrix}


\makeatletter
\ExplSyntaxOn
\NewDocumentCommand \WhenNotMeasuring { } { \legacy_if:nF {measuring@} }
\makeatother
\ExplSyntaxOff


\begin{document}

\begin{align*}
    \begin{pNiceMatrix}[name=myMatrix]
        1 & 1 & 1 \\
        2 & 2 & 2 \\
    \end{pNiceMatrix}
    \WhenNotMeasuring
      {
        \begin{tikzpicture}[remember picture, overlay]
            \draw (myMatrix-1-1) -- (myMatrix-1-2);
        \end{tikzpicture}
      }
\end{align*}

\end{document}

Answer 2

align* environment typesets its contents twice, and I believe this causes your problem.

Recent version of nicematrix package provides a command \CodeAfter which is helpful in your case. Note that you can even omit the [name=myMatrix] part.

\documentclass{article}
\usepackage{amsmath}
\usepackage{nicematrix}
\usepackage{tikz}

\begin{document}
\begin{align*}
    \begin{pNiceMatrix}
        1 & 1 & 1 \\
        2 & 2 & 2 
        \CodeAfter
        \tikz \draw (1-1) -- (1-2);
    \end{pNiceMatrix}
\end{align*}
\end{document}