How to patch samepage for svmono?

Question

Continuing Why doesn't \samepage set \predisplaypenalty (to \@M or any other value)? , let's say we wish to patch LaTeX's

\DeclareRobustCommand\samepage{\interlinepenalty\@M
   \postdisplaypenalty\@M
   \interdisplaylinepenalty\@M
   \@beginparpenalty\@M
   \@endparpenalty\@M
   \@itempenalty\@M
   \@secpenalty\@M
   \interfootnotelinepenalty\@M}

by means of adding \predisplaypenalty\@M somewhere (e.g., because your classes or packages (say, svmono.cls) set \predisplaypenalty to a value less than \@M.) We do not plan to use \samepage directly but rather \begin{samepage}…\end{samepage} Which way to go?

My attempts have failed so far. Here's one example of failure:

\documentclass{svmono}% version 5.10 (08-Sep-21); cf. http://www.springer.com/gp/authors-editors/book-authors-editors/your-publication-journey/manuscript-preparation#toc-49268 or http://resource-cms.springernature.com/springer-cms/rest/v1/content/20566/data/monographs
% at this spot we wish to redefine the samepage environment, but this one doesn't work:
\usepackage{etoolbox}
\makeatletter
\patchcmd{\samepage}{\interfootnotelinepenalty\@M}{\interfootnotelinepenalty\@M\predisplaypenalty\@M}{}{\typeout{samepage could not be redefined}}
\makeatother
\usepackage{amssymb,mathtools,cleveref}
\begin{document}
\begin{samepage}
  \strut\vskip120ex
  \begin{remark}[A remark name]\label[remark]{label}
    \[
      \text{a formula}
    \]
    \begin{center}%
      Table caption\\%%% probably better \nopagebreak\\ instead of \\, but anyway, samepage should better do all the job itself.
      \hbox to \hsize{\hfill
        \begin{tabular}[b]{|c|}
          \hline
          Some table\\
          \hline
        \end{tabular}%
        \hfill\llap{$\square$}}%
    \end{center}%
  \end{remark}%
\end{samepage}
\end{document}

yields

samepage could not be redefined

in the log and a page break in the remark:

How to properly™ extend or redefine the samepage environment such that we don't get a page break inside the remark?

Answer 1

etoolbox cannot patch commands defined using \DeclareRobustCommand. You can use package xpatch to append code to a robust command with \xapptocmd. Here you can see, that the patch is used:

\documentclass{svmono}% version 5.10 (08-Sep-21); cf. http://www.springer.com/gp/authors-editors/book-authors-editors/your-publication-journey/manuscript-preparation#toc-49268 or http://resource-cms.springernature.com/springer-cms/rest/v1/content/20566/data/monographs
% at this spot we wish to redefine the samepage environment, but this one doesn't work:
\usepackage{xpatch}
\makeatletter
\xapptocmd{\samepage}{\predisplaypenalty\@M}{}{\PatchFailure}
\makeatother
\usepackage{amssymb,mathtools,cleveref}
\begin{document}
\begin{samepage}
  \strut\vskip120ex
  \begin{remark}[A remark name]\label[remark]{label}
    \verb|\predisplaypenalty| is \the\predisplaypenalty
    \[
      \text{a formula}
    \]
    \begin{center}%
      A table\\%%% probably better \nopagebreak\\ instead of \\, but anyway, samepage should better do all the job itself.
      \hbox to \hsize{\hfill
        \begin{tabular}[b]{|c|}
          \hline
          Some table\\
          \hline
        \end{tabular}%
        \hfill\llap{$\square$}}%
    \end{center}%
  \end{remark}%
\end{samepage}
\end{document}

But if you have a look at the whole thing, you can also see, that this does not prevent the page break after the remark head:

And even, if you mode the \begin{samepage} after \strut\vskip120ex you would get the same result

So the root of your problem is not the \predisplaypentalty. If you really don't want a page break inside a remark, I would suggest to use a minipage:

\documentclass{svmono}% version 5.10 (08-Sep-21); cf. http://www.springer.com/gp/authors-editors/book-authors-editors/your-publication-journey/manuscript-preparation#toc-49268 or http://resource-cms.springernature.com/springer-cms/rest/v1/content/20566/data/monographs
% at this spot we wish to redefine the samepage environment, but this one doesn't work:
\usepackage{amssymb,mathtools,cleveref}
\begin{document}
  \strut\vskip120ex
\noindent\begin{minipage}{\textwidth}
  \begin{remark}[A remark name]\label[remark]{label}
    \[
      \text{a formula}
    \]
    \begin{center}%
      A table\\%%% probably better \nopagebreak\\ instead of \\, but anyway, samepage should better do all the job itself.
      \hbox to \hsize{\hfill
        \begin{tabular}[b]{|c|}
          \hline
          Some table\\
          \hline
        \end{tabular}%
        \hfill\llap{$\square$}}%
    \end{center}%
  \end{remark}%
\end{minipage}
\end{document}

If you only not want a page break at the beginning of the remark, use package needspace.