A zero \parskip and an unusual \paragraph result in overlapping paragraphs

Question

I'm trying to debug a class provided by some journal. From times to times, the text appears completely garbled, or does not appear at all (I believe it is printed somewhere but probably out of the page). This class is a complete (and useless) reimplementation of article, but I think I narrowed down the problem.

Consider the following document:

 \documentclass[a4paper,10pt,twoside]{article}
 \usepackage{lipsum}

 \makeatletter
 \renewcommand\paragraph{\@startsection{paragraph}
                               {4}{\z@}{12pt \@plus -3pt}{-.5em}
                               {\normalfont\normalsize\bfseries}}
 \parskip\z@
 \makeatother

 \begin{document}
 \lipsum[1-3]
 \paragraph{New Par} \footnote{Oh}\lipsum[1-2]
 \end{document}

While I believe that the \paragraph definition is inoffensive it seems that, in combination with the \parskip\z@, the results get wrong:

Question: What fix should I give to the editor? Don't set \parskip to zero? Is that really the problem? Is the problem in the definition of \paragraph?

Answer 1

The problem is with the definition of \paragraph: the fourth argument ("beforeskip") contains a negative plus component, which it shouldn't. Changing this to 12pt \@plus 3pt fixes it.

A negative stretch (or shrink) is hardly ever useful, except for certain cases.

Usually, I would leave it at that – but because I suspect that there is a reason to why this negative stretch is there (and because you've asked for it ...), it might be worthwhile to delve into the intricacies of \@startsection:

According to source2e, the "beforeskip" argument is to specify the following:

Absolute value = skip to leave above the heading. If negative, then paragraph indent of text following heading is suppressed.

Now, if latex encounters a negative value (of the first part: e.g., -12pt), it will (1) reverse the sign of the skip (which means reversing the sign of all components of this skip: its natural width, stretch and shrink), and (2) set the paragraph indentation switch to false. So that, for example, -12pt \@plus -3pt \@minus -3pt would actually mean: beforeskip of 12pt \@plus 3pt \@minus 3pt and "no indentation of the next paragraph". Since negative stretch or shrink components usually don't make much sense (see above), all components should be either positive or negative, but not mixed.

The fifth argument ("afterskip") specifies:

if positive, then skip to leave below heading, else negative of skip to leave to right of run-in heading.

In your definition of \paragraph, the fifth argument is negative, signalling that it should be a run-in heading, and not start a new paragraph. Therefore, it actually does not matter whether "beforeskip" is positive or negative, as text following a run-in heading is of course never indented.