Box taking exactly N \baselineskip

Question

I'm trying to define a memoir's \fancybreak taking a vertical space of, say, 2\baselineskip (or any other exact amount of \baselineskip).

But I have a problem with the height: if I do

\documentclass[twocolumn]{memoir}
\usepackage{lipsum}

\parindent=0pt

\begin{document}

\lipsum[1-4]

\vbox to 2\baselineskip{
\vspace*{\fill}
\centering *
\vspace*{\fill}
}

\lipsum[2]

\end{document}

I get a space of something more than 2\baselineskip:

Why? Which approach would you suggest?

Answer 1

Positioning vertical boxes can be tricky, it's easier to use horizontal boxes, which more naturally stay on the line grid.

\documentclass[twocolumn]{memoir}
\usepackage{lipsum}

\parindent=0pt

\begin{document}

\lipsum[1-4]

\noindent
\mbox{}\\[-.5\baselineskip]
\makebox[\linewidth]{*}\\[-.5\baselineskip]
\mbox{}

\lipsum[2]

\end{document}

Answer 2

Using David's idea, I create n horizontal boxes. In the last one I insert a zero height \vbox, which in turn contains the material to be vertically and horizontally centered. The asterisks are those of math mode, so we can lower them by the axis height and smash them.

Some additional vertical space has to be added before the material, because of the gap between the baseline of the last \hbox and the text in the following line. I used \dp\strutbox, which seems to provide a decent centering (the wye will not be misled anyway).

\documentclass[twocolumn]{memoir}
\usepackage{lipsum}
\usepackage{xcolor}

\ExplSyntaxOn

\NewDocumentCommand{\interruption}{mm}
 {% #1 = number of lines
  % #2 = material to be centered
  \par
  \prg_replicate:nn { #1 - 1 } { \hbox:n { } }
  \hbox:n
   {
    \vbox_to_zero:n
     {
      \vss
      \vbox_to_ht:nn { #1\baselineskip }
       {
        \vfil
        \vspace{\dp\strutbox}
        \hbox_to_wd:nn { \linewidth } { \hfil #2 \hfil }
        \vfil
       }
     }
   }
 }

\ExplSyntaxOff

% asterisks centered on the baseline
\NewDocumentCommand{\asterisks}{}{%
  \sbox{0}{$ $}\raisebox{-\fontdimen22\textfont2}[0pt][0pt]{${*}{*}{*}$}%
}

% this will show the baseline
\newcommand{\debugrule}{\makebox[0pt]{%
  \color{red}\vrule width 3\textwidth height 0pt depth 0.3pt}%
}

\begin{document}

\lipsum[1]

\interruption{3}{\asterisks}

\debugrule
\lipsum[2-4]

\interruption{2}{\asterisks}

\debugrule
\lipsum[2]

\end{document}

With \debugrule I show that the baselines in the two columns agree.