latex tikz/pgfkeys

Question

I have the following code

\documentclass[10pt, a4paper]{article}

\usepackage{nicematrix}
\usepackage{tikz}
\usetikzlibrary{calc}


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

\let\adding\relax
\pgfkeys{
    /adding/.code=\ifx\adding\relax
    \edef\adding{#1}%
    \else
    \edef\adding{\adding, #1}%
    \fi
}

\begin{document}
\begin{align*}
    \begin{bNiceMatrix}[name=mymatrix]
        1 & 2 & 3 \\
        4 & 5 & 6 \\
        7 & 8 & 9 \\
    \end{bNiceMatrix}
    \WhenNotMeasuring{
        \begin{tikzpicture}[remember picture,overlay]
            \pgfkeys{/adding={2-3/3-3/2}}
            \foreach \x/\y/\z in \adding{
                \draw[->] (mymatrix-\x) ++(.75em,0) -- ++(.75em,0) node [anchor=west]{\tiny x\z} |- ($ (mymatrix-\y) + (0.75em,0) $);
            }
            %\foreach \x/\y in {\pgfkeysvalueof{/mult}}
            %\draw (\x) ++(.75em,0) node [anchor=west]{\tiny \y};
            %\foreach \x/\y in {\pgfkeysvalueof{\swap}}
            %\draw (\x) -- ++(3ex,0) |- (\y);
        \end{tikzpicture}
        }
\end{align*}

\end{document}

But now if I move the \pgfkeys{/adding={2-3/3-3/2}} to the nicematrix env e.g.

\begin{bNiceMatrix}[name=mymatrix]
        1 & 2 & 3 \\
        4 & 5 & 6 \\ \pgfkeys{/adding={1-3/2-3/2}}
        7 & 8 & 9 \\
    \end{bNiceMatrix}

The arrow isn't drawn anymore.

Any Ideas why and how to solve it?

EDIT:

\documentclass[10pt, a4paper]{article}

\usepackage{nicematrix}
\usepackage{tikz}
\usetikzlibrary{calc}


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

\makeatletter
\pgfkeys{
    /adding/.code=\ifx\tikz@atticus@adding\relax
    \xdef\tikz@atticus@adding{#1}%
    \else
    \xdef\tikz@atticus@adding{\tikz@atticus@adding, #1}%
    \fi,/tikz/.cd,
    reset Gauss/.code=\global\let\tikz@atticus@adding\relax,
    reset Gauss,
    get list/.code=\edef#1{\tikz@atticus@adding},
    get list/.default=\adding,
}
\makeatother

\newenvironment{gaussMatrix}%
{
    \begin{pNiceMatrix}[name=mymatrix]
}%
{
\end{pNiceMatrix}
\WhenNotMeasuring{
    \begin{tikzpicture}[remember picture,overlay,get list=\adding]
        \foreach \x/\y/\z in \adding
        \draw[->] (mymatrix-\x) ++(.75em,0) -- ++(.75em,0) node [anchor=west]{\tiny x\z} |- ($ (mymatrix-\y) + (0.75em,0) $);
        \tikzset{reset Gauss}% <-important
    \end{tikzpicture}
}
    }


    \begin{document}
    \begin{align*}
        \begin{gaussMatrix}
            1 & 1 & 1 & 1 & 1 \\
            2 & 2 & 2 & 2 & 2 \\
            3 & 3 & 3 & 3 & 3 \\
        \end{gaussMatrix}
    \end{align*}

    \end{document}

Answer 1

You can solve the problem by making your macro global. Usually this is something that one should avoid.¹ If one really wants to go that way, one typically introduces macros that contain some special character, traditionally an @ with \makeatletter. Then one has to be careful to reset the list, which was not necessary with local definitions. To this end, I added a reset Gauss key. So here is a prototype example.

\documentclass[10pt, a4paper]{article}

\usepackage{nicematrix}
\usepackage{tikz}
\usetikzlibrary{calc}


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

\makeatletter
\pgfkeys{
    /adding/.code=\ifx\tikz@atticus@adding\relax
    \xdef\tikz@atticus@adding{#1}%
    \else
    \xdef\tikz@atticus@adding{\tikz@atticus@adding, #1}%
    \fi,/tikz/.cd,
    reset Gauss/.code=\global\let\tikz@atticus@adding\relax,
    reset Gauss,
    get list/.code=\edef#1{\tikz@atticus@adding},
    get list/.default=\adding,
}
\makeatother
\begin{document}
\begin{align*}
    \begin{bNiceMatrix}[name=mymatrix]
        1 & 2 & 3 \\
        4 & 5 & 6 \\ \pgfkeys{/adding={2-3/3-3/2}}
        7 & 8 & 9 \\
    \end{bNiceMatrix}
    \WhenNotMeasuring{
        \begin{tikzpicture}[remember picture,overlay,get list=\adding]            
            \foreach \x/\y/\z in \adding{
                \draw[->] (mymatrix-\x) ++(.75em,0) -- ++(.75em,0) node [anchor=west]{\tiny x\z} |- ($ (mymatrix-\y) + (0.75em,0) $);
            }
        \tikzset{reset Gauss}% <-important
        \end{tikzpicture}
        }
\end{align*}

\end{document}

This does work. However, as you got a couple of excellent answers by the author of the nicematrix, F. Patigny, I think the cleaner way might be if one could build in some official hooks in the nicematrix package (in case they do not exist already) to have cleaner, safer code. I am writing this merely because you asked in the comments.

_{¹Future versions of pgf will be able to solve the problem to some extent: https://tex.stackexchange.com/a/491246.}