Changing inner sep of tikz matrix without affecting the nodes inside it

Question

This is related to Baseline of tikz picture within brackets but is a separate issue.

Consider the following MWE:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix} % for "matrix of math nodes"
\begin{document}
\begin{equation}
    \begin{tikzpicture}[baseline=(current bounding box.center)]
        \matrix[matrix of math nodes, draw, nodes={draw}] {
            a & b & c \
            d & e & f \
        };
    \end{tikzpicture}
\end{equation}
\end{document}

It produces this output:

I would like to get rid of the space between the nodes inside the matrix and the bounding box of the matrix. Setting the matrix's inner sep to 0 achieves this --- but it also changes the inner sep of all the nodes inside the matrix, which isn't what I want:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix} % for "matrix of math nodes"
\begin{document}
\begin{equation}
    \begin{tikzpicture}[baseline=(current bounding box.center)]
        \matrix[matrix of math nodes, draw, nodes={draw}, inner sep=0pt] {
            a & b & c \
            d & e & f \
        };
    \end{tikzpicture}
\end{equation}
\end{document}

How can I change the inner sep of the matrix node while leaving the nodes inside it alone?

I found a near-duplicate question, How to fix a certain distance between a TikZ matrix border and its nodes? . However, the answer to that question just sets the inner sep of the nodes explicitly, which I want to avoid - I want to just leave each node's inner sep at whatever value it would have if it wasn't in the matrix.

Answer 1

With the style /tikz/every outer matrix you can change the style of only the matrix node instead of the matrix's nodes.

Code

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix} % for "matrix of math nodes"
\tikzset{tight matrix/.style={every outer matrix/.append style={inner sep=+0pt}}}
\begin{document}
\begin{equation}
    \begin{tikzpicture}[baseline=(current bounding box.center)]
        \matrix[matrix of math nodes, draw, nodes={draw}, tight matrix] {
            a & b & c \\
            d & e & f \\
        };
    \end{tikzpicture}
\end{equation}
\end{document}

Output

Answer 2

To my knowledge there is no inner sep parameter for the matrix node. So, when certain inner sep is fixed it decides the inner separation of nodes, but also the separation between nodes and matrix border. To avoid this behaviour, you should use M. Al Jumaily's solution: nodes={inner sep=...}.

If you don't liker it, a possible alternative could be to not draw the matrix border and draw it with a fit node. In this case, you keep original nodes inner sep (initially .3333em) and can draw the border to whatever separation you decide.

Following code shows this solution. The matrix border is not draw and three different borders are added with fit nodes.

I think it's not a good solution because you have to distinguish between real matrix node and fitting node and you have to know which inner nodes are larger to include them inside fit list.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix, fit} % for "matrix of math nodes"
\begin{document}
\begin{equation}
    \begin{tikzpicture}[baseline=(current bounding box.center)]
        \matrix[matrix of math nodes, nodes={draw}] (A) {
            a & b & c \
            d & e & f \
        };
     \node[fit={(A-1-1) (A-1-2) (A-2-3)}, inner sep=0pt, draw=green, inner sep=2pt] (B) {};
     \node[fit={(A-1-1) (A-1-2) (A-2-3)}, inner sep=0pt, draw=blue, inner sep=0pt] (C) {};
     \node[fit={(A-1-1) (A-1-2) (A-2-3)}, inner sep=0pt, draw=red, inner sep=5mm] (D) {};
    \end{tikzpicture}
\end{equation}
\end{document}

Answer 3

This answer is based on Fritz's answer as credit should always be given.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}
\tikzset{custom nodes/.style={
    draw=red,
    matrix of math nodes,
    cells/.expanded={
        nodes={draw=blue},
        inner xsep=\pgfkeysvalueof{/pgf/inner xsep},
        inner ysep=\pgfkeysvalueof{/pgf/inner ysep}
    }
}}
\begin{document}
    \begin{equation}
        \begin{tikzpicture}[ baseline=(current bounding box.center)]
        \matrix[custom nodes, inner sep=0pt] {
            a & b & c \
            d & e & f \
            };
        \end{tikzpicture}
    \end{equation}
\end{document}