As a follow-up to my previous Question French section numbering using bis, ter, etc, I'm attempting to manually add line breaks in my section titles that show up in the body of the text as intended but do not break manually in the Table of Contents. I hope this is clear - if not, the screencap below should hopefully show what I'm after:

This I achieved using manual section numbering, which is what I sought to avoid in the first Question. The syntax for that is as follows:
\documentclass{book}
\usepackage{xparse}
\usepackage{enumitem}
\usepackage{titlesec}
\usepackage{lipsum}
\usepackage{titletoc}
\titleformat{\section}[block]{\normalfont\normalsize\itshape\filcenter}{\thesection. ---}{0.5em}{}
\renewcommand\thesection{\arabic{section}}
\begin{document}
\tableofcontents
\section{First section header}
\lipsum[1]
\section*{1 \textit{bis}. --- Second section header is very long \\ and I want to control the location where it breaks}\addcontentsline{toc}{section}{1 \textit{bis} Second section header is very long and I want to control the location where it breaks}
\lipsum[4]
\end{document}
Here's my non-working MWE, based on the syntax mvienney came up with in the aforementioned Question. Note that I did not use this syntax to come up with the image above. If I try to put line breaks in the section titles for this syntax, it fails to compile.
\documentclass{book}
\usepackage{xparse}
\usepackage{enumitem}
\usepackage[explicit]{titlesec}
\usepackage{lipsum}
\usepackage{titletoc}
\ExplSyntaxOn
\seq_new:N \bislist
\seq_set_split:Nnn \bislist {;} {bis;ter;quater;quinquies;sexies;septies;octies;novies;decies}
\NewDocumentCommand {\bisprint} {m}
{
\seq_item:Nn \bislist {#1}
}
\ExplSyntaxOff
\newcounter{bis}
\stepcounter{bis}
\titleformat{\section}[block]{\normalfont\normalsize\itshape\filcenter}{\thesection. --- #1}{0.5em}{}[\setcounter{bis}{1}]
\renewcommand\thesection{\arabic{section}}
\titleformat{name=\section,numberless}[block]{\normalfont\normalsize\itshape\filcenter}{\arabic{section} \textit{\bisprint{\arabic{bis}}}. --- #1}{0.5em}{}[ \addcontentsline{toc}{section}{\numberline{\arabic{section} \textit{\bisprint{\arabic{bis}}}}#1} \stepcounter{bis}]
\makeatletter
\renewcommand*\l@section{\@dottedtocline{1}{1.5em}{3.5em}} %change the last parameter to 2.5em if you do not go further that ter and to 3.5 if you don't go further than quater
\makeatother
\begin{document}
\tableofcontents
\section{First section header}
\lipsum[1]
\section*{Second section header is very long and I want to control the location where it breaks}
\lipsum[4]
\end{document}

tocloftfrom the start?! Also, I think it's better to usetitletocsince you're usingtitlesec. – Werner Jul 16 '15 at 14:35