Plain TeX section number in margin

Question

I want to typeset the number for the section in the margin. Here's an MWE of what I'm starting from.

%  --- Fonts ------------------------------------------------
\font\0="Roboto Condensed Light" at 12pt\0
\font\ssa="Roboto Condensed Bold" at 24pt

%  --- Layout ------------------------------------------------
\raggedright
\parindent=0pt
\parskip=1em

%  --- Structure ---------------------------------------------
\newcount\SectionNumber

\def\Section #1\par{%
\vskip 40pt
\advance \SectionNumber by 1
{\ssa\the\SectionNumber. #1}
\vskip 8pt
}

% Start Text
\Section Project One

\input knuth

\bye

I'm looking at this answer and trying to adapt it here, so I came up with this:

\def\inmargin#1{\strut\vadjust{\kern-\dp\strutbox\smash{\llap{#1}}\kern\dp\strutbox}}

\def\Section #1\par{%
\vskip 40pt
\advance \SectionNumber by 1
{\ssa\inmargin{\the\SectionNumber. }#1}
\vskip 8pt
}

This is close, but when the section header has descenders it seems to be incorrectly aligned. See:

As you can see, the 1. is a little below where it should be. The problem doesn't occur if I omit the descender:

I tried various contortions creating my own box and moving it around, and also trying to just kern the whole section heading negatively into the margin. I don't really care what approach is taken, I'd just like to understand what I'm missing. I suspect it has something to do with the height and depth of the section heading text.

Answer 1

Let's see what happens with your \inmargin macro:

\def\inmargin#1{%
  \strut
  \vadjust{
    \kern-\dp\strutbox
    \smash{\llap{#1}}%
    \kern\dp\strutbox
  }%
}

You emit a \strut that starts horizontal mode, because it does \unhcopy\strutbox. However, you should be aware of the fact that Plain does

\setbox\strutbox=\hbox{\vrule height8.5pt depth3.5pt width0pt}

which is quite small, compared to \ssa which is a font at 24pt. Then you tell TeX that, after the paragraph has been typeset, after the first line of the paragraph (which is just the section title), a vertical kern of -\dp\strutbox (which is -3.5pt), a smashed box containing the section number and another kern (of 3.5pt) will be inserted. Now, if the section title has a descender it will be way more than 3.5pt, because it uses a large font.

The macro you are trying to modify is for adding something in the left margin without knowing the point where this addition is made. In your case you do know when the object in the margin must be announced: at the \Section command. So no trick is necessary: just \llap it.

However, \llap does \hbox, which is not a horizontal command, so you need to start it in some way. The correct way is to issue \noindent, that works independently of your setting to \parindent.

\def\Section#1\par{%
  % be in vertical mode
  \par
  % step the section number
  \global\advance\SectionNumber by 1
  % don't bother to add vertical space if there's already
  \ifdim\lastskip<40pt \removelastskip\penalty-400 \vskip40pt\fi
  % start horizontal mode without indent
  \noindent
  % print the section number
  \llap{\ssa\the\SectionNumber. }%
  % print the section title
  {\ssa\baselineskip=32pt \interlinepenalty=10000 #1\par}%
  % no page break
  \nobreak
  % some vertical space
  \vskip 8pt
}

Full example

%  --- Fonts ------------------------------------------------
\font\roboto="Roboto Condensed Light" at 12pt
\roboto
\font\ssa="Roboto Condensed Bold" at 24pt

%  --- Layout ------------------------------------------------
\raggedright
\parindent=0pt
\parskip=1em

%  --- Structure ---------------------------------------------
\newcount\SectionNumber

\def\Section#1\par{%
  % be in vertical mode
  \par
  % step the section number
  \global\advance\SectionNumber by 1
  % don't bother to add vertical space if there's already
  \ifdim\lastskip<40pt \removelastskip\penalty-400 \vskip40pt\fi
  % start horizontal mode without indent
  \noindent
  % print the section number
  \llap{\ssa\the\SectionNumber. }%
  % print the section title
  {\ssa\baselineskip=32pt \interlinepenalty=10000 #1\par}%
  % no page break
  \nobreak
  % some vertical space
  \vskip 8pt
}

% Start Text
\Section Project One

\input knuth

\Section Project Two

\input knuth

\bye

Answer 2

You should be in horizontal mode when setting the sectional title. The following should work just fine:

\def\Section #1\par{%
\vskip 40pt
\advance \SectionNumber by 1
{\leavevmode\ssa\llap{\the\SectionNumber. }#1}
\vskip 8pt
}

Note the use of \leavevmode to enter horizontal mode.