Shrinking padding of node with label options

Question

I am trying to implement command — let's name it fn — aimed to behave like overset command and underline the lower word with some curve. Doing this using TikZ. The lower word is made as node text, and the upper one — as node's label option. And because of that some additional padding is appearing like in MWE below.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{xparse}
\newcommand{\tmpText}{}

\NewDocumentCommand{\fn}{ > { \SplitArgument { 1 } { : } } m }{%
    {%
        \NewDocumentCommand{\tmpSubject}{mm}{%
            \IfValueT{##2}{\renewcommand{\tmpText}{##2}}
            \tikz[baseline=(Root.base)]{%
                \node[inner sep=0pt,
                      outer sep=0pt,
                      label={[yshift=-2,
                              font=\tiny\itshape] \tmpText}] (Root) {##1};
                \draw[line width=0.75]
                    let
                        \p1 = (Root.south west),
                        \p2 = (Root.south east)
                    in
                        (\x1, \y1 - 2) -- (\x2, \y2 - 2);
            }%
        }%
        \tmpSubject#1%
    }%
}


\begin{document}
{\setlength{\fboxsep}{0pt}
    \fbox{\fn{Hello:world}}
    \fbox{\fn{Hello}}
    \fbox{Hello}
}
\end{document}

Which is producing the following output:

So, the main question is whether it is possible to correct paddings so that the output would be as follow (color is only for indicating desired area)? And of course, if the upper text is wider than lower one, width as well should be corrected to the wider text. Alas, could not find the answer myself neither on forum nor in TikZ.

And the additional question is it a good way to define temporary commands inside another command in terms of performance? Like I defined fn in MWE? My main motivation of such nested macros is not to define function I do not directly need in main document.

Thanks in advance.

Answer 1

You correctly set the inner sep to 0 for the node but forgot to do so for the label, which is also a node. Once you set it to 0 (or 0.5pt for the optics) there, you no longer need the negative yshift. I also added a %, and marked the position with <-.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{xparse}
\newcommand{\tmpText}{}

\NewDocumentCommand{\fn}{ > { \SplitArgument { 1 } { : } } m }{%
    {%
        \NewDocumentCommand{\tmpSubject}{mm}{%
            \IfValueT{##2}{\renewcommand{\tmpText}{##2}}% <-
            \tikz[baseline=(Root.base)]{%
                \node[inner sep=0pt,
                      outer sep=0pt,
                      label={[inner sep=0.5pt,
                              font=\tiny\itshape] \tmpText}] (Root) {##1};
                \draw[line width=0.75]
                    let
                        \p1 = (Root.south west),
                        \p2 = (Root.south east)
                    in
                        (\x1, \y1 - 2) -- (\x2, \y2 - 2);
            }%
        }%
        \tmpSubject#1%
    }%
}


\begin{document}
{\setlength{\fboxsep}{0pt}
    \fbox{\fn{Hello:world}}
    \fbox{\fn{Hello}}
    \fbox{Hello}
}
\end{document}

Or without calc but with line cap=rect.

\documentclass{article}

\usepackage{tikz}
\usepackage{xparse}
\newcommand{\tmpText}{}

\NewDocumentCommand{\fn}{ > { \SplitArgument { 1 } { : } } m }{%
    {%
        \NewDocumentCommand{\tmpSubject}{mm}{%
            \IfValueT{##2}{\renewcommand{\tmpText}{##2}}%
            \tikz[baseline=(Root.base)]{%
                \node[inner sep=0pt,
                      outer sep=0pt,
                      label={[inner sep=0.5pt,
                              font=\tiny\itshape] \tmpText}] (Root) {##1};
                \draw[line width=0.75,line cap=rect]
                        ([yshift=-2pt]Root.south west)--
                        ([yshift=-2pt]Root.south east);
            }%
        }%
        \tmpSubject#1%
    }%
}


\begin{document}
{\setlength{\fboxsep}{0pt}
    \fbox{\fn{Hello:world}}
    \fbox{\fn{Hello}}
    \fbox{Hello}
}
\end{document}

Or, a third version where we do not allow TikZ to choose the bounding box according to a path that has line cap=rect. To this end, I only insert a coordinate at the bottom and draw the line overlay, i.e. exclude it from the bounding box. This version comes with a comparison with plain \fboxes.

\documentclass{article}

\usepackage{tikz}
\usepackage{xparse}
\newcommand{\tmpText}{}

\NewDocumentCommand{\fn}{ > { \SplitArgument { 1 } { : } } m }{%
    {%
        \NewDocumentCommand{\tmpSubject}{mm}{%
            \IfValueT{##2}{\renewcommand{\tmpText}{##2}}%
            \tikz[baseline=(Root.base)]{%
                \node[inner sep=0pt,
                      outer sep=0pt,
                      label={[inner sep=0.5pt,
                              font=\tiny\itshape] \tmpText}] (Root) {##1};
                \path ([yshift=-2.375pt]Root.south west);             
                \draw[line width=0.75,overlay]
                        ([yshift=-2pt]Root.south west)--
                        ([yshift=-2pt]Root.south east);
            }%
        }%
        \tmpSubject#1%
    }%
}

\newcommand{\cmd}[1]{\texttt{\textbackslash #1}}
\begin{document}
{\setlength{\fboxsep}{0pt}
\begin{tabular}{|@{\,}l@{\,}|c|@{}l@{}|}
\fbox{\fn{Hello:world}} & \cmd{fd} with label & \fbox{\fn{Hello:world}}\\
\fbox{Hello}  & just \cmd{fbox} for comparison & \fbox{Hello}\\
\fbox{\fn{Hello}}  & \cmd{fd} without label & \fbox{\fn{Hello}} \\
\fbox{Hello}  & just \cmd{fbox} for comparison & \fbox{Hello}\\
\end{tabular}}
\end{document}

Let me also draw your attention to the tikzmark library, which comes with the command \tikzmarknode, which detect its ambient text (math mode vs. text mode etc.), which may be of use here.