How can I prevent LGR encoding from switching font size?

Question

I am using 12pt fonts with what is essentially the article class. (That is, I'm using a custom class which is parasitic on article.cls.)

In order to set an appropriate headheight for fancyhdr using \geometry{}, I test the font size at the end of the preamble using \AtEndPreamble{} from etoolbox. The height is then adjusted according to whether 10pt, 11pt or 12pt is being used.

This all works fine with the following code:

\documentclass[12pt]{article}
\usepackage{etoolbox}
\usepackage[T1]{fontenc}
\makeatletter
\AtEndPreamble{%
  \PackageWarning{mine}{font size is \f@size}
}
\makeatother

\begin{document}

some text

\end{document}

The log contains:

Package mine Warning: font size is 12 on input line 13.

However, if I need to typeset Greek, I want to use:

\documentclass[12pt]{article}
\usepackage{etoolbox}
\usepackage[LGR,T1]{fontenc}
\makeatletter
\AtEndPreamble{%
  \PackageWarning{mine}{font size is \f@size}
}
\makeatother

\begin{document}

some text

\end{document}

Only now the log contains:

Package mine Warning: font size is 10 on input line 13.

Why does loading the LGR encoding change the font size at all? And why does it change it even when T1 is the default encoding?

How can I prevent or work around this problem?

Note that I am aware that I could use XeTeX or LuaTeX or ConTeXt to avoid this. However, for better or worse, this question is about a (pdf)TeX solution.

If there is an alternative way of typesetting small amounts of Greek text using (pdf)TeX, that would be perfectly satisfactory. I use the above with utf8 and inputenc which allows me to input the odd Greek phrase in unicode characters and have everything typeset nicely. Anything which supports that would work fine.

This question seemed related but I am using TeX Live and already have the relevant type1 fonts installed (e.g. /usr/local/texlive/2014/texmf-dist/fonts/type1/public/cbfonts/grmn1200.pfb).

Answer 1

You can rely on the font size only after \normalsize has been issued. This is part of \begin{document}.

I see no problem in setting \headheight as part of

\AtBeginDocument{...}

rather than \AtEndPreamble. However, doing

\AtEndPreamble{%
  \normalsize
  \PackageWarning{mine}{font size is \f@size}
}

prints

Package mine Warning: font size is 12 on input line 11.

in the .log file.

Answer 2

The file lgrenc.def includes the line

\DeclareErrorFont{LGR}{cmr}{m}{n}{10}

and it's this that causes the change of font size. This command resets \f@size amongst other things (taken from ltfssbas.dtx):

%  \begin{macro}{\DeclareErrorFont}
%    Declare the last resort shape! We assume that in this fontshape
%    there is a 10pt font but it doesn't really matter. We only loose
%    one macro name if the assumption is false. But at least the font
%    should be there!
%    \begin{macrocode}
\def\DeclareErrorFont#1#2#3#4#5{%
      \xdef\error@fontshape{%
          \noexpand\expandafter\noexpand\split@name\noexpand\string
          \expandafter\noexpand\csname#1/#2/#3/#4/#5\endcsname
          \noexpand\@nil}%
%    \end{macrocode}
%    Initialize all those internal variables which may or may not have
%    values in the first seconds of NFSS' bootstraping process. Later
%    on such values will be updated when an encoding is selected, etc.
%
%    We definitely don't want to set |\f@encoding|; we can set all the
%    others since if they are left ``blank'' any selection would grap
%    ``error default values'' as well. However, this probably should
%    go also.
% \changes{v2.1n}{1994/05/14}{Don't set \cs{f@encoding}}
%    \begin{macrocode}
%      \gdef\f@encoding{#1}%
      \gdef\default@family{#2}%
      \gdef\default@series{#3}%
      \gdef\default@shape{#4}%
      \global\let\f@family\default@family
      \global\let\f@series\default@series
      \global\let\f@shape\default@shape
      \gdef\f@size{#5}%
      \gdef\f@baselineskip{#5pt}%
}
\@onlypreamble\DeclareErrorFont
%    \end{macrocode}
%  \end{macro}

As you can see, this is based on the idea some other reset will take place later.

Other encodings don't alter the fall-back, so you never see the fact that this also resets the size.

I guess in terms of 'how do I prevent this', you would have to locally disable \DeclareErrorFont (not ideal!).

\documentclass[12pt]{article}
\usepackage{etoolbox}
\let\savedDeclareErrorFont\DeclareErrorFont
\def\DeclareErrorFont#1#2#3#4#5{}
\usepackage[LGR,T1]{fontenc}
\let\DeclareErrorFont\savedDeclareErrorFont
\makeatletter
\AtEndPreamble{%
  \PackageWarning{mine}{font size is \f@size}
}
\makeatother

\begin{document}

some text

\end{document}