Extra space on the left of Forest tree

Question

I made a wide tree with Forest and I want use resizebox to scale it down to match the text length. I noticed that the space on the left of the tree is oddly bigger, specially if compared to the space on the right. How I can fix it?

\documentclass{article}

\usepackage{graphicx}
\usepackage{forest}

\begin{document}

Some text
\begin{center}
        \resizebox{\textwidth}{!}{
            \fbox{             
                \begin{forest}
                    for tree = {l sep = 20 mm}
                    [Root
                        [A long label for a inner node
                            [A long label for a leaf]
                            [A long label for a leaf]
                        ]
                        [A long label for a inner node
                            [A long label for a leaf]
                            [A long label for a leaf]
                        ]
                        [A long label for a inner node
                            [A long label for a leaf]
                            [A long label for a leaf]
                        ]
                        [A long label for a inner node
                            [A long label for a leaf]
                            [A long label for a leaf]
                        ]
                    ]
                \end{forest}
            }
        }
\end{center}
other text.

\end{document} 

Here is the output:

Answer 1

A hack: write this after loading forest.

\makeatletter
\preto\forest@pack{\nullfont}
\makeatother

This makes forest switch to the null font before starting to pack. (Packing is the heart of forest, where the positions of nodes are computed. There's a lot of computation going on there. In particular, a lot of line intersections are computed, using PGF's machinery, and this is where we typically get the leaking spaces.)

While the hack eats up the leaking spaces, it introduces an empty line (maybe a paragraph break?) on top of \fbox. I don't know why precisely this happens (I hope somebody can explain). A hack within hack is to add the comment signs at the beginning of \resizebox and \fbox arguments -- this removes the space tokens.

\documentclass{article}

\usepackage{graphicx}
\usepackage{forest}

\makeatletter
\preto\forest@pack{\nullfont}
\makeatother

\begin{document}

Some text
\begin{center}
        \resizebox{\textwidth}{!}{% without these comments
            \fbox{% we get some extra vertical space
                \begin{forest}
                    for tree = {l sep = 20 mm}
                    [Root
                        [A long label for a inner node
                            [A long label for a leaf]
                            [A long label for a leaf]
                        ]
                        [A long label for a inner node
                            [A long label for a leaf]
                            [A long label for a leaf]
                        ]
                        [A long label for a inner node
                            [A long label for a leaf]
                            [A long label for a leaf]
                        ]
                        [A long label for a inner node
                            [A long label for a leaf]
                            [A long label for a leaf]
                        ]
                    ]
                \end{forest}
            }
        }
\end{center}
other text.

\end{document} 

Note that while the hack works in this particular example, it will not do in general. It is sure to fail if you use typeset node after packing, because the null font is still in effect.

If I manage to iron out these kinks, I might even consider integrating it into forest itself -- after all, it is inavoidable that some future version of PGF will introduce some new leaking spaces ...

Answer 2

Better than make note text unreadable with scaling of forest tree diagram is consider to rotate it with grow=east and (if the nodes text is really long) enables nodes' texts to break into more lines. For example:

\documentclass{article}
%---------------- show page layout. don't use in a real document!
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%

\usepackage{forest}

\begin{document}

Some text
\begin{center}
\fbox{
    \begin{forest}
        for tree = {
        font=\small,
        grow'=east,
        child anchor=west,
        l sep = 12 mm,
        s sep =2mm,
        where level = {0}{}{text width=width("A long label ")}
                    }
        [Root
            [A long label for a inner node
                [A long label for a leaf]
                [A long label for a leaf]
            ]
            [A long label for a inner node
                [A long label for a leaf]
                [A long label for a leaf]
            ]
            [A long label for a inner node
                [A long label for a leaf]
                [A long label for a leaf]
            ]
            [A long label for a inner node
                [A long label for a leaf]
                [A long label for a leaf]
            ]
        ]
    \end{forest}
}
\end{center}
other text.

\end{document} 

(red lines indicate text borders)