[texhax] A question about stretchability and shrinkability

[Oleg Katsitadze]

On Tue, Jun 16, 2009 at 11:36:21AM +0100, Rodolfo Medina wrote:
> \def\subchapter#1{%
>   \vskip0pt plus.3\vsize\vskip0pt plus-.3\vsize
>   \bigskip\bigskip\vskip\parskip
>   \leftline{#1}
>   \vskip0pt plus.3\vsize\vskip0pt plus-.3\vsize
>   \bigskip\vskip\parskip
>   \par\nobreak\noindent\ignorespaces
>      \vtop to 0pt
>      \bgroup
>      \vskip -25pt
>      <some stuff>
>      \vss
>      \egroup
>   \ignorespaces
> }
>
>
> Now, what I noticed is that the \vskip that I set to -25pt slightly varies, in
> the output, from place to place.
>
> Why does this happen, and what should put instead to prevent that lenght to
> change?

I don't know how you came to the conclusion that it's the \vskip-25pt
glue that changes, but I'm pretty sure that if you examine the output
of \showlists you'll see that the glue is -25pt everywhere.  What
probably changes is the glue before the \vtop, which displaces the
entire vbox depending on how much TeX has decided to stretch/shrink
vertical glue on that page.

HTH,
Oleg