How to restore normal flow after \filbreak

Question

\documentclass[a4paper]{article}
\usepackage{lipsum}
\begin{document}
  \lipsum[1-4]\filbreak
  \textbf{This should fit on page 1}~\lipsum[5]\filbreak\lipsum[6-10]\filbreak
  \textbf{This should not fit on page 2}~\lipsum[11-12]\filbreak\lipsum[13-19]
\end{document}

The problem arises with the last \filbreak in this example. It flushes \lipsum[13-19] to the next page. I want to replace the last \filbreak it with a command \stopfilbreak, so that since \lipsum[11-12] actualy didn't fit on page 2, then it would put \lipsum[11-12] on page 3 with \lipsum[13-19] following it (on page 3). Now it sets \lipsum[13-19] on page 4.

I want a command \stopfilbreak such that LaTeX interprets "fit the content between \filbreak and \stopfilbreak on the same page if possible, else break the page here and put it on a new page". The content to come after \stopfilbreak should not be affected.

That is: i want a command that restores the normal flow after use of \filbreak

---

Be aware that it might be so that what would go between \filbreak and \stopfilbreak could be larger than a pagewidth, so minipage would not work.

Below is some background info that I think might be helpful to answer my question:

---

Background info

Purpose of \filbreak

The TeXbook says that \filbreak means roughly

“Break the page here and fill the bottom with blank space, unless there is room for more copy that is itself followed by \filbreak.”

That is, if we consider content that is "enclosed" by \filbreaks, it inserts this content if there is room on the page, but goes to the next page if there is no room.

\filbreak in practice

\filbreak is defined by \par\vfil\penalty-200\vfilneg. As I understand it, this finishes up the current paragraph, then fills the page with stretchable whitespace and then, at the bottom of the page/top of new page it says that "breaking" here is cheap. If the program then decides to break here, then the \vfilneg doesn't do anything because there is no \vfil to "neg", but if it doesn't then \vfilneg puts the "cursor" back.

---

I'm not using \filbreak as it was purposed. Everything in my VC-example works as expected, which indicates that I might be asking the wrong question?

Answer 1

I had previously posted, and subsequently deleted, an answer that was completely wrong; now I’m replacing it with the following one, which not only makes at least some sense, but also seems to work as expected! (:-)

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{lipsum}

\makeatletter

\@ifdefinable\@My@filbreak@counter{\newcount\@My@filbreak@counter}
\global \@My@filbreak@counter = 0
\@ifdefinable\@My@filbreak@mark{\newmarks\@My@filbreak@mark}

\renewcommand*\filbreak{%
    \par
    \global \advance \@My@filbreak@counter \@ne
    \marks\@My@filbreak@mark{\number\@My@filbreak@counter}%
    \vfil
    \penalty -200
    \vfilneg
}

\newcommand*\@My@finish@stopfilbreak{}

\newcommand*\stopfilbreak{%
    \par
    \ifnum \prevdepth >\@m \else \prevdepth \z@ \fi
    \nobreak \null
    \nobreak \vskip -\baselineskip
    \penalty \z@
    \begingroup
        \output{%
            % \typeout{>>> \topmarks\@My@filbreak@mark
            %         \space vs \number\@My@filbreak@counter}%
            \typeout{t: \the\pagetotal, g: \the\pagegoal}%
            \unvbox\@cclv
            \ifnum 0\topmarks\@My@filbreak@mark <\@My@filbreak@counter
                % the last "\filbreak" is still on the current page
                \gdef\@My@finish@stopfilbreak{%
                    \vfil\break % force page break
                }%
            \else
                % the page has changed since the last "\filbreak"
                \gdef\@My@finish@stopfilbreak{%
                    \penalty\z@ % allow page break but don't force it
                }%
            \fi
        }%
        \break
    \endgroup
    \@My@finish@stopfilbreak
}

\makeatother

\newcommand*{\mylipsum}[1]{\par\textbf{#1}~-- \lipsum*[#1]\par}

% \tracingpages = 1



\begin{document}
    \mylipsum{1}
    \filbreak
    \mylipsum{2}
    \filbreak
    \mylipsum{3}
    \filbreak
    \mylipsum{4}
    \filbreak
    \mylipsum{5}
    \filbreak
    \mylipsum{6}
    \filbreak
    \mylipsum{7}
    \filbreak
    \mylipsum{8}
    \mylipsum{9}
    \mylipsum{10}
    % \mylipsum{11}
    \indent One.\par
    \indent Two.\par
    \indent Three.\par
    \indent Four.\par
    % \indent Five.\par
    % \indent Six.\par
    % \mylipsum{12}
    \stopfilbreak
    \mylipsum{13}
    \mylipsum{14}
    \mylipsum{15}
    \mylipsum{16}
\end{document}

Try uncommenting the line that says % \mylipsum{11}, or the line that reads % \indent Five.\par.

I’m too weary, now, to explain in detail how the code works; perhaps I’ll review this answer tomorrow, for now I simply hint at the general idea. We use a custom \marks system, namely \marks\@my@filbreak@mark, to detect if the last \filbreak before \stopfilbreak occurs on the same page as the latter, and if so we force a page break; otherwise, \stopfilbreaks inserts only a permissible breakpoint.

I’ve left the previous version of this answer accessible through the edit list so that everyboby can contemplate how incredibly stupid it was…