How to prevent TeX from stretching glue beyond a given maximum?

Question

Is it possible to define a glue that TeX will not stretch any further than a given limit--under no circumstances?

In the following MWE, I want to set a piece of text apart from the rest by half a line, and in case that piece of text lies on a page break, that half line should be allowed to be stretched to one full line, so that the whole text stays on the baseline grid and fills the page. However, I never want the glue to stretch more than one line, no matter if the page would become underfull or not.

\documentclass{memoir}

\usepackage{tikz, lipsum, everypage}

% show the baseline grid
\makeatletter
\newcommand{\left@margin}{}
\AddEverypageHook{
    \ifodd\value{page}\relax%
        \renewcommand{\left@margin}{\spinemargin}
    \else
        \renewcommand{\left@margin}{\foremargin}
    \fi
    % + \trimtop because it is also "\trimbottom"
    \tikz[overlay, remember picture, background rectangle/.style={fill=none},
            shift={(current page.south west)},xshift={\left@margin + \trimedge},
            yshift={\lowermargin + \trimtop}]%
    {%
        \draw[red!20!white,thin] (0,0) grid[step=\onelineskip] (\textwidth,
        \textheight);%
        \draw[red,thin] (0,0) rectangle (\textwidth,\textheight);
    }%
}%
\makeatother

\parskip=0pt

\begin{document}

\lipsum*[1-2]

\vskip .5\baselineskip plus .5\baselineskip % HERE should be the .5 limit

This is a test line of text to show my case, and I needed a few words to fill the line.

\vskip .5\baselineskip plus .5\baselineskip % HERE should be the .5 limit

\lipsum*[1-2]  Morbi auctor lorem non justo. Nam lacus libero, pretium at, lobortis
Morbi auctor lorem non justo. Nam lacus libero, pretium at, lobortis Morbi auctor
lorem non justo. Nam lacus libero, pretium at, lobortis Nam lacus libero, pretium at,
lobortis Morbi auctor lorem non justo. Nam

\section{Test}

\lipsum*[1]

\end{document}

The result looks like this:

You can see that the two lines including the space before and after it occupy 5 lines instead of just 4 lines.

Answer 1

If the stretch component is non zero then the glue will stretch arbitrarily large (but with increasing badness) so if you want an upper limit then the plus component has to be zero. You can however use a length of

1\baselineskip minus 0.5\baselineskip

which is restricted to the finite range that you specify, however its natural length is the upper rather than lower limit, so you may need to "encourage" it to shrink.

Alternatively you can use fixed units and not allow stretch or shrink

\vskip 1\baselineskip
\penalty0
\vskip-0.5\baselineskip

will be a half baselineskip unless a page break happens at the penalty, in which case it will be a full baselineskip (with the -0.5\baselineskip glue discarded at the break)