Double-spaced paragraphs with single-spaced headers in classicthesis?

Question

I've been writing my PhD thesis using the classicthesis template with the arsclassica style package. My university requires that I double-space the body of the thesis, so I'm using \doublespacing for most of the document. This has the annoying effect of double-spacing my chapter and section headings as well. I could manually insert \singlespacing before and \doublespacing after every chapter/section/subsection heading, but this would be very tedious.

I'm aware of several previous answers on this topic, in particular this one from @GonzaloMedina, which suggests two possible remedies:

1. Use etoolbox to insert \singlespacing before, and \doublespacing after, each sectional unit:

   \usepackage{etoolbox}
   \makeatletter
   \pretocmd{\@sect}{\singlespacing}{}{}
   \pretocmd{\@ssect}{\singlespacing}{}{}
   \apptocmd{\@sect}{\doublespacing}{}{}
   \apptocmd{\@ssect}{\doublespacing}{}{}
   \makeatother
   

   Unfortunately this doesn't seem to have any effect for classicthesis documents.

2. Use titlesec to specify the formatting of the headings directly:

   \usepackage{titlesec}
   \titleformat{\section}
   {\singlespacing\normalfont\Large\bfseries}{\thesection}{1em}{}
   \titleformat{\subsection}
   {\singlespacing\normalfont\large\bfseries}{\thesubsection}{1em}{}
   \titleformat{\subsubsection}
   {\singlespacing\normalfont\normalsize\bfseries}{\thesubsubsection}{1em}{}
   

   However, this overrides the nice default formatting provided by arsclassica/classicthesis.

In the comments section under that question, @PeterGrill suggests using

\let\OldSection\section
\renewcommand*{\section}[1]{\singlespace\OldSection{#1}\doublespace}

This is the closest I currently have to a working solution, but it unfortunately prevents me from using the optional \section[Short title]{Long title} syntax, which would be useful for some of my long section headings. He also suggested another variant to address this:

\usepackage{letltxmacro}
\LetLtxMacro\OldSection\section
\renewcommand*{\section}[2][]{\singlespace\OldSection[#1]{#2}\doublespace}

But, as Gonzalo pointed out, this prevents the TOC from being generated correctly.

In short, I'd like to know whether there is a better way to specifically enforce single spacing for chapter/section/subsection headings within an otherwise double-spaced document that uses classicthesis/arsclassica.

---

Below is a simple test-case:

\documentclass[headinclude, oneside]{scrbook}
\usepackage[eulerchapternumbers,beramono,pdfspacing]{classicthesis}
\usepackage{setspace}
\usepackage{lipsum}

\begin{document}
\doublespacing
\section[A short section name]{A long section name that spans multiple lines within the document but should be single-spaced}
\lipsum[1]
\section[Another short section name]{Another long section name that spans multiple lines within the document but should be single-spaced}
\lipsum[2]
\end{document}

Answer 1

Redo the relevant \titleformat command:

\documentclass[headinclude, oneside]{scrbook}
\usepackage[eulerchapternumbers,beramono,pdfspacing]{classicthesis}
\usepackage{arsclassica}
\usepackage{setspace}
\usepackage{lipsum}

\titleformat{\section}
  {\singlespacing}
  {\textsc{\MakeTextLowercase{\thesection}}}
  {1em}
  {\spacedlowsmallcaps}


\begin{document}
\doublespacing

\section[A short section name]{A long section name that spans multiple lines within the document but
should be single-spaced}

\lipsum[1]

\section[Another short section name]{Another long section name that spans multiple lines within the
document but should be single-spaced}

\lipsum[2]
\end{document}

With or without arsclassica is the same.