Missing characters in output due to `\renewcommand{\familydefault}{\sfdefault}`

Question

This is a followup question on: ASCII text set in Greek script when using \usepackage[greek]{babel}

When we have a MWE like:

\documentclass[twoside]{book}

\usepackage{fontspec}
\usepackage[greek]{babel}
\usepackage{alphabeta}
\setmainfont{Libertinus Serif}

\renewcommand{\familydefault}{\sfdefault}

\begin{document}

{My Project }

{Δημιουργήθηκε από  Doxygen 1.8.19}

\end{document}

we get:

We see that here part of the second line is missing, we expected:

This result is obtained by commenting out the line: \renewcommand{\familydefault}{\sfdefault} so an easy solution would be to do this also in the production code, but I hesitate as this line has been added in 2013 and it is unknown why it is added (doxygen was not yet in git at that moment, so individual commits were nor registered).

To give a bit side information, it looks like that the following lines were added at the same moment as well:

\usepackage[T1]{fontenc}
\usepackage[scaled=.90]{helvet}
\usepackage{courier}
\usepackage{amssymb}
\usepackage{sectsty}
\renewcommand{\familydefault}{\sfdefault}
\allsectionsfont{%
  \fontseries{bc}\selectfont%
  \color{darkgray}%
}

• any idea why such a line\renewcommand{\familydefault}{\sfdefault} could have been added?
• any suggestions for improvement of this line \renewcommand{\familydefault}{\sfdefault} so the functionality remains but the side effect disappears?

Answer 1

You seem to want a document with a sans serif font as default:

\documentclass[twoside]{book}

\usepackage{fontspec}
\usepackage[greek]{babel}
%\usepackage{alphabeta} % why?
\setmainfont{Libertinus Sans}

\begin{document}

My Project

Δημιουργήθηκε από  Doxygen 1.8.19

\end{document}

Use the font you'd like best, provided it supports Greek.

With your code you set the serif font to Libertinus Serif, but since you don't specify the sans serif font family, the redefinition of \familydefault to \sffamily will choose the default, that is, Latin Modern Sans, which doesn't support Greek.

I don't think that alphabeta is necessary.

Answer 2

The Computer Modern Unicode fonts are clones of Knuth’s Computer Modern, like Latin Modern is, that contain Greek. So you could use

\setmainfont{CMU Serif}
\setsansfont{CMU Sans Serif}
\setmonofont{CMU Typewriter Text}

A good tip for solving bugs like this is to add \tracinglostchars=2 to the top of the file. Then TeX will tell you if the font you selected does not contain a character you’re using. By default, it silently logs a warning message to the .log file.