[texhax] strange behavior of amsmath

[jfbu]

Le 12 déc. 2013 à 00:43, Reinhard Kotucha <reinhard.kotucha at web.de> a écrit :

> Hi,
> I would expect that the fleqn option of amsmath has only an impact on
> horizontal glue.
> 
> Curiously, if the fleqn option is active, there is extra vertical glue
> below the first formula of each paragraph and below the first formula
> following ordinary text.
> 
> Here is a minimal example:
> 
> <testfleqn.tex.gz>
> You get correct vertical spacing if you comment out the line
> containing "[fleqn]".  The relevant part of the amsmath source code
> looks quite innocent.  I can reproduce the problem with TeX Live 2003
> too, the oldest TL release I've currently installed.
> 
> The vertical bar in the test file is an argument of \smash, hence
> both, its height and depth is zero.  Its sole purpose is to make the
> extra glue more visible.
> 
> Please also note that "some text" in the test file doesn't start a new
> paragraph.
> 
> Where does the extra vertical space come from?  I doubt that it's
> deliberate. 
> 
> Regards,
>  Reinhard
> 
> 

As an aside, using

\documentclass
% [fleqn]
{article}
% commenting out amsmath:
% \usepackage
% [fleqn]
% {amsmath}

with then the rest of Reinhard's code one observes also 
a somewhat disturbing vertical spacing: in the non-numbered
display the first formula uses the long skips

with class option fleqn however, everything uses consistently
long skips. As the difference between long and short display
skips is much less pronounced in LaTeX than in TeX (I admit
this contradicts my earlier rant against LaTeX's vertical
skips), this looks fine although a bit more spread out
than without fleqn class option

Regards,
Jean-Francois