How do you effectively debug overfull \hbox warnings?

Question

I still haven't found out how to effectively solve Overfull \hbox (0.80913pt too wide) has occurred while \output is active warnings.

For instance, this piece of latex generates such a warning and I am stumped why:

\newcommand\Header{%
\noindent\makebox[\textwidth][l]{
\parbox{0.5\textwidth}{\contactinfo}
\parbox{0.5\textwidth}{%
\hfill\includegraphics[height=25pt]{logo}}}\par\vskip1ex%
\par\ColTopRule\par}

\contactinfo is defined as follows:

\newcommand\contactinfo{\fontsize{8pt}{8}\selectfont\headerfnt Streetname 12324 \\ PostalCode City \\ +31655555555555}

Error:

*geometry detected driver: pdftex*
<logo.eps>
Overfull \hbox (0.80913pt too wide) has occurred while \output is active
 [] 
[1] <logo.eps>
Overfull \hbox (0.80913pt too wide) has occurred while \output is active
 [] 
[2] <logo.eps>
Overfull \hbox (0.80913pt too wide) has occurred while \output is active
 [] 
[3] <logo.eps>
Overfull \hbox (0.80913pt too wide) has occurred while \output is active
 [] 

Expected output is:

Streetname 12324                                       here comes
PostalCode City                                          the logo
+31655555555555                                     right aligned
-----------------------------------------------------------------

I mean, both boxes have a length of 0.5\textwidth, the text is obviously smaller than this, as is the logo. How come it still generates such a warning? And how can I found out why?

Answer 1

To answer the general question about how to debug the box warnings add

\showboxdepth=\maxdimen
\showboxbreadth=\maxdimen

or if that is too much information

\showboxdepth=5
\showboxbreadth=5

and then whenever TeX is showing box information in the log it will show you a representation of the contents of the box.

Answer 2

Some other ways:

• The answer in https://tex.stackexchange.com/a/493311/250119 shows how to append a message at the end of overfull hboxes.

  The output looks something like this:

  

• The answer in How to visualize the underfull box in final output PDF files similarly uses Lua callback to mark underfull vboxes in red. It is probably adaptable.

• lua-visual-debug package can be used to look at the boxes, although unfortunately this by itself does not show which box is overfull.

Answer 3

Another method is to look at the code and know. For example, the code presented in the question:

\makebox[\textwidth][l]{              % it is \hbox to\hsize and it starts with space here
\parbox{0.5\textwidth}{\contactinfo}  % it creates a box with 0.5\hsize width and there is second space here
\parbox{0.5\textwidth}{%              % it creates a box with 0.5\hsize width 
\hfill\includegraphics[height=25pt]{logo}}}

Two boxes with 0.5\hsize plus two spaces is more than one \hsize.