Unexpected linebreak with makebox

Question

I think the picture explains my Problem very well:

The chapter enumeration on odd pages should behave the same as all other titles (e.g. subtitle on picture). I can't figure out why it is not working. Maybe someone has an idea?

Here's my code:

\documentclass[twoside]{scrreprt}
\usepackage{blindtext}

% Align titles to outer side
\renewcommand*{\raggedsection}{\ifthispageodd{\raggedleft}{\raggedright}}

% Push section enumeration outward
\renewcommand*{\sectionlinesformat}[4]{%
  \ifthispagewasodd
    #4\makebox[0pt][l]{\hskip\marginparsep #3}%
  \else
    \makebox[0pt][r]{#3\hskip\marginparsep}#4%
  \fi
}

% Push chapter enumeration outward 
\makeatletter
\renewcommand*{\chapterlinesformat}[3]{%
    \ifthispagewasodd
        #3\makebox[0pt][l]{\hskip\marginparsep #2}% % THE ERROR SHOULD BE HERE?
    \else
        \makebox[0pt][r]{#2\hskip\marginparsep}#3%
    \fi
}
\makeatother

\renewcommand*{\chapterformat}{\thechapter\autodot}
\renewcommand*{\sectionformat}{\thesection\autodot}
\renewcommand*{\subsectionformat}{\thesubsection\autodot}
\renewcommand*{\subsubsectionformat}{\thesubsubsection\autodot}

\begin{document}
\chapter{Another Chapter}
\subsection{Subsection}
\blindtext
\chapter{Another Chapter}
\subsection{Subsection}
\blindtext
\end{document}

Answer 1

If I add \showtokens{#3} in the body of the redefinition of \chapterlinesformat, I get

> \interlinepenalty \@M Another Chapter\@@par .

and this explains why the number ends up in the next line: \par (the primitive one) is added at the end of the chapter title.

We have to remove the \@@par and reinsert it where it belongs.

\documentclass[twoside]{scrreprt}
\usepackage{blindtext}

% Align titles to outer side
\renewcommand*{\raggedsection}{\ifthispageodd{\raggedleft}{\raggedright}}

% Push section enumeration outward
\renewcommand*{\sectionlinesformat}[4]{%
  \ifthispagewasodd
    #4\makebox[0pt][l]{\hskip\marginparsep #3}%
  \else
    \makebox[0pt][r]{#3\hskip\marginparsep}#4%
  \fi
}

% Push chapter enumeration outward 
\makeatletter
\renewcommand*{\chapterlinesformat}[3]{%
    \ifthispagewasodd
        \remove@@par#3\makebox[0pt][l]{\hskip\marginparsep #2}\@@par
    \else
        \makebox[0pt][r]{#2\hskip\marginparsep}#3%
    \fi
}
\def\remove@@par#1\@@par{#1}
\makeatother

\renewcommand*{\chapterformat}{\thechapter\autodot}
\renewcommand*{\sectionformat}{\thesection\autodot}
\renewcommand*{\subsectionformat}{\thesubsection\autodot}
\renewcommand*{\subsubsectionformat}{\thesubsubsection\autodot}

\begin{document}
\chapter{Another Chapter}
\subsection{Subsection}
\blindtext
\chapter{Another Chapter}
\subsection{Subsection}
\blindtext
\end{document}