Flafter sometimes ignores a section title

Question

I'm using the flafter package to prevent floats from appearing before their positions in the LaTeX source code.

When I put a float just after a section title and the section starts at the beginning of the page, sometimes the float appears above the section title, on the top of the page, while it should appear under the section title.

In the example below:

• by uncommenting paragraph2 the float will be under the section title (as expected);
• by commenting paragraph2 the float will be above the section title (at the top of the page).

I'd like to put the float under the section title in both cases.

\documentclass{article}
\usepackage{mwe}

\usepackage{flafter}

\begin{document}
\blindtext[4]

paragraph1

%paragraph2

\section{My section}
\begin{figure}[htb]
\includegraphics{example-image-a}
\end{figure}

\blindtext
\end{document}

Answer 1

The problem is actually independent of flafter, it happens even if you comment out the package altogether: as the section appears on page 2 why is the "h" float not after the section but on top of it?

The reason for this behavior is that a \section command attempts to keep the material following it on the same page. Therefore while LaTeX is still processing page 1 it looks at the \section and at the float following it and only then finally decides that the section doesn't fit on the first page.

However, by that time it has also concluded that the float doesn't fit either so it has been put on the deferlist. And from there it makes it way happily into the top area of the next page (in case of flafter because obviously (?), something that got deferred must be after its callout).

Guess that assumption is wrong so yes that is a bug or a deficiency in flafter but not sure that is something that can easily be fixed. In normal circumstances (i.e., when a float is placed between paragraphs this is not someting that would happen unless you set very very weird paragraph penalties (e.g., be favoring widow lines over finishing a paragraph). It only happens if LaTeX has look far ahead and ends up not using any material direcly in front of the float, which is the case with the section setup.

Answer 2

That looks like a potential bug, but as a workaround you can force the page break where the page break finally happened, before the section, then the float placement gets in sync:

\documentclass{article}
\usepackage{mwe}

\usepackage{flafter}

\begin{document}
\blindtext[4]

paragraph1

%paragraph2
\clearpage
\section{My section}

\begin{figure}[hbpt]
\includegraphics{example-image-a}
\end{figure}

\blindtext
\end{document}