largestsep does not work as advertised

[RuixiZhang42]

Brief description of the bug

With package option largestsep, titlesec does not use the larger skip, but instead use the sum of <after-sep> from the first heading and <before-sep> from the second heading.

MWE demonstrating the issue

MWE adapted from https://tex.stackexchange.com/q/269484 with more exaggerated spacing:

\documentclass{article}
\usepackage{lipsum}
\usepackage[largestsep]{titlesec}
\titlespacing{\section}   {0pt}{20pt}{20pt}
\titlespacing{\subsection}{0pt}{20pt}{10pt}
\begin{document}
Expected:

\vskip20pt

\Large Section

\vskip20pt

\large Subsection

\vskip10pt

\normalsize \lipsum[1][1-5]

\noindent
\hrulefill

Actual:
\section{Section}
\subsection{Subsection}
\lipsum[1][1-5]
\end{document}

Possible fixes

titlesec uses \vspace to add <after-sep>, and this is the bug.

In the LaTeX2e kernel \vspace{#1} expands to ... \vskip #1 \vskip\z@skip ..., so when the next <before-sep> appears in \ttl@titlespace{<before-sep>} (since \let\ttl@titlespace\addvspace in this case), the \addvspace acts on \z@skip and not the intended <after-sep>.

[jbezos]

Thank you for the report and for the fix (and sorry for the late answer). I'll try to release a new version it in a few weeks.