What does manyfoot do to multicol that fnpara does not?

Question

If I use the fnpara package for paragraph footnotes:

\documentclass{article}
\usepackage{multicol}
\usepackage{lipsum}
\usepackage{fnpara}
\begin{document}
\begin{multicols}{2}
\lipsum[1]
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\footnote{This is a test.}\footnote{This is a test.}\footnote{This is a test.}
\lipsum[1-10]
\end{multicols}
\end{document}

then the footnotes overlap footer.footmisc package suffers from the same issue. However this does not happen with manyfoot package

\documentclass{article}
\usepackage{multicol}
\usepackage{lipsum}
\usepackage[para]{manyfoot}
\DeclareNewFootnote[para]{A}
\begin{document}
\begin{multicols}{2}
\lipsum[1]
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\footnoteA{This is a test.}\footnoteA{This is a test.}\footnoteA{This is a test.}
\lipsum[1-10]
\end{multicols}
\end{document}

What does manyfoot package do to multicol that fnpara does not? how can this issue be fixed with fnpara and footmisc packages?

Answer 1

fnpara and footmisc are using the same algorithm (more or less) originally from an example by Don Knuth in the TeXbook. At some point they combine all collected footnotes on the page and unbox them into a paragraph (command doing that is called \makefootnoteparagraph) The width of that paragraph is explicitly set in the macro as

 \hsize=\columnwidth

That is usually correct in LaTeX as in two-column mode you want such a paragraph be the width of the column. However within multicols the expectation is that footnotes span all columns so in that case the paragraph should be set to \textwidth.

So a possible workaround is to patch that command to use \textwidth instead, i.e.,

\usepackage{etoolbox}
\patchcmd{\makefootnoteparagraph}
   {\columnwidth}{\textwidth}
   {\typeout{Success}}{\typeout{patch failed}}

which works for both fnpara and footmisc.

But note that this is not a universal patch, i.e., it would be wrong if you do not use multicol, but instead use \twocolumn or the twocolumn option to the class!

P.S. not checked how it is done in manyfoot. That packages does much more elaborate maniplations and probably uses a different algorithm to get paragraph footnotes and therefore doesn't run into the problm.