Unexplained Overfull Box

Question

I was trying to create a long horizontal bar where to put the marks of an assessment and I had created the following code.

\documentclass{article}
\usepackage{tikz}
\setlength{\parindent}{0pt}
\newcommand{\TotalMarks}[1]{%
    \tikz\draw[thick]
    (0,0) -- (\linewidth,0) -- 
    ++(0,2em) -- ++(-2em,0) -- 
    ++(0,-2em) node[pos=0.6,left] 
        {\makebox[2.9cm]{\textbf{Total: #1 marks}\hfill}};}
\begin{document}
\TotalMarks{2}
\end{document}

I cannot understand why it gives an Overfull error:

Overfull \hbox (0.79999pt too wide) in paragraph at lines 14--15[][] 

How can I fix it?

Thank you for any help you may give me.

Answer 1

There are two things that cause this:

1. Paragraph indentation. The standard paragraph indentation is added before the tikzpicture, you can disable that for a single line with \noindent, i.e. \noindent\tikz...

2. When you draw a line, the bounding box will actually extend by half the line width beyond the end coordinates, as you can see from this example:

    \documentclass[border=5mm]{standalone}
    \usepackage{tikz}
    \begin{document}
   \begin{tikzpicture}
      \draw [line width=5mm] (0,0) -- (1,0);
      \draw [very thin, red] (0,-0.5) -- (0,0.5);
   \draw [thin, blue] (current bounding box.south east) rectangle (current bounding box.north west);
    \end{tikzpicture}
    \end{document}
   

The blue line indicates the bounding box.

If you add line cap=rect, the line will extend to fill that blank space, and you pgf has the handy macro \pgflinewidth that has the width of the current path. Hence, you can do \draw[thick,line cap=rect] (0,0) -- (\linewidth-\pgflinewidth,0) ....

Working example, as egreg mentioned you shouldn't use minimal (Why should the minimal class be avoided?):

\documentclass{article}
\usepackage{tikz}
\newcommand{\TotalMarks}[1]{%
    \tikz\draw[thick,line cap=rect]
    (0,0) -- (\linewidth-\pgflinewidth,0) -- 
    ++(0,2em) -- ++(-2em,0) -- 
    ++(0,-2em) node[pos=0.6,left] 
        {\makebox[2.9cm]{\textbf{Total: #1 marks}\hfill}};}
\begin{document}
\noindent\TotalMarks{2}
\end{document}