Chapter Section Subsection headings in one page alignment

Question

I want to align the texts after the numbers (6, 6.1, 6.1.1) in (Chapter, Section, Subsection) titles in one page. But I can only find the alignments of the Numbers for example left aligned.

I have now the new problem. It works for the headings of Chapter, Section and Subsection and also Subsubsection. The heading of Subsubsection does not have numbes. How to make the text of Subsubsection align with the number of Chapter or Section or Subsection?

\documentclass
  [openright,listof=totoc,bibliography=totoc,twoside,fontsize=12pt,
   english,parskip=half,headinclude,footinclude=false,headsepline,
   DIV17,BCOR16mm,numbers=noenddot,2.1headlines,appendixprefix,
   cleardoublepage=empty
  ]{scrbook}

Answer 1

Here is a suggestion that aligns the texts only for the whole document (and not per page). And you have to set the space needed for the section numbers manually.

\documentclass{scrbook}[2015/10/03]
\newdimen\sectionnumberwdmax
\setlength\sectionnumberwdmax{2cm}% <- space needed for the section numbers
%\renewcommand\raggedsection{\raggedleft}
\makeatletter
\renewcommand\sectionlinesformat[4]{%
  \@hangfrom%
    {\makebox[\sectionnumberwdmax][l]{\hskip#2#3}}%
    {\parbox[t]{\dimexpr\textwidth-\sectionnumberwdmax\relax}{\raggedsection #4}}%
}
\renewcommand\chapterlinesformat[3]{%
  \@hangfrom%
  {\makebox[\sectionnumberwdmax][l]{#2}}%
  {\parbox[t]{\dimexpr\textwidth-\sectionnumberwdmax\relax}{\raggedsection #3}}%
}
\makeatother

\begin{document}
\chapter{A chapter}
\section{A section}
\subsection{A subsection}
\end{document}

---

If unnumbered titles should be aligned with the numbers of the other titles:

\documentclass{scrbook}[2015/10/03]
\newdimen\sectionnumberwdmax
\setlength\sectionnumberwdmax{2cm}% <- space needed for the section numbers
%\renewcommand\raggedsection{\raggedleft}
\makeatletter
\renewcommand\sectionlinesformat[4]{%
  \ifstr{#3}{}
    {\@hangfrom{\hskip#2#3}{#4}}
    {\@hangfrom%
      {\makebox[\sectionnumberwdmax][l]{\hskip#2#3}}%
      {\parbox[t]{\dimexpr\textwidth-\sectionnumberwdmax\relax}{\raggedsection #4}}}%
}
\renewcommand\chapterlinesformat[3]{%
    \ifstr{#2}{}
    {\@hangfrom{#2}{#3}}
    {\@hangfrom%
      {\makebox[\sectionnumberwdmax][l]{#2}}%
      {\parbox[t]{\dimexpr\textwidth-\sectionnumberwdmax\relax}{\raggedsection #3}}}%
}
\makeatother

\begin{document}
\tableofcontents
\chapter{A chapter}
\section{A section}
\subsection{A subsection}
\subsubsection{A subsubsection}
\end{document}

Answer 2

This works per page, but needs at least two LaTeX runs after adding a new heading:

\documentclass{scrbook}
\makeatletter
\AtBeginDocument{%
  \if@filesw
    \write\@auxout{%
      \noexpand\providecommand*\noexpand\minnumwidth[3]{}%
    }%
  \fi
}
\newcommand*{\minnumwidth}[3]{%
  \scr@ifundefinedorrelax{maxnumwidth@page@#2}{%
    \expandafter\gdef\csname maxnumwidth@page@#2\endcsname{#3}%
  }{%
    \expandafter\ifdim \csname maxnumwidth@page@#2\endcsname<#3\relax
      \expandafter\gdef\csname maxnumwidth@page@#2\endcsname{#3}%
    \fi
  }%
  \expandafter\gdef\csname pageof@#1\endcsname{#2}%
}
\newcommand*{\makenumberboxof}[1]{%
  \scr@ifundefinedorrelax{pageof@#1}{%
    #1\autodot\enskip
  }{%
    \scr@ifundefinedorrelax{maxnumwidth@page@\@nameuse{pageof@#1}}{%
      #1\autodot\enskip
    }{%
      \makebox[\@nameuse{maxnumwidth@page@\@nameuse{pageof@#1}}][l]%
      {#1\autodot\enskip}%
    }%
  }%
}
\newcommand*{\changenumformat}[1]{%
  \expandafter\renewcommand\expandafter*\csname #1format\endcsname{%
    \begingroup
      \settowidth{\@tempdima}{\csname the#1\endcsname\autodot\enskip}%
      \edef\reserved@a{\string\minnumwidth{\csname
          the#1\endcsname}{\noexpand\thepage}{\the\@tempdima}}%
      \if@filesw
        \expandafter\write\expandafter\@auxout\expandafter{\reserved@a}%
      \fi
    \endgroup
    \makenumberboxof{\csname the#1\endcsname}%
  }%
}
\makeatother
\changenumformat{chapter}
\changenumformat{section}
\changenumformat{subsection}
\changenumformat{subsubsection}
\makeatother

\usepackage{blindtext}

\begin{document}
\blinddocument\blinddocument\blinddocument\blinddocument\blinddocument
\blinddocument\blinddocument\blinddocument\blinddocument\blinddocument
\blinddocument\blinddocument\blinddocument\blinddocument\blinddocument
\chapter{A chapter}
\section{A section}
\subsection{A subsection}

\appendix
\chapter{An appendix}
\section{An appendix section}
\subsection{An appendix subsection}
\end{document}

After the first LaTeX run the headings of the last page are:

but after the third run they are:

Note that it does work only as long as the numbers of chapters, sections and subsections are expandable.

Not numbered headings are still left aligned. So, if I understand your addendum to the question correct, nothing has to be done for those. But if I don't and if this per page solution should also be used for not numbered headings, you have to add additional code, e.g., for the \subsubsection:

\usepackage{xpatch}
\newcounter{notnumberedsection}
\renewcommand*{\thenotnumberedsection}{\arabic{notnumberedsection}*}
\makeatletter
\xpretocmd{\subsubsection}{%
  \let\scr@subsubsection@sectionindent\z@
  \ifnumbered{subsubsection}{%
    \typeout{subsubsection numbered!}%
  }{%
    \stepcounter{notnumberedsection}%
    \if@filesw
      \edef\reserved@a{\string\minnumwidth{\thenotnumberedsection}{\noexpand\thepage}{\string\z@}}%
      \show\reserved@a
      \expandafter\write\expandafter\@auxout\expandafter{\reserved@a}%
    \fi
    \scr@ifundefinedorrelax{pageof@\thenotnumberedsection}{%
      \typeout{Don't know page of \thenotnumberedsection!}%
    }{%
      \scr@ifundefinedorrelax{maxnumwidth@page@\@nameuse{pageof@\thenotnumberedsection}}{%
        \typeout{Don't know max width for page \@nameuse{pageof@\thenotnumberedsection}!}%
      }{%
        \expandafter\let\expandafter\scr@subsubsection@sectionindent
        \csname
        maxnumwidth@page@\@nameuse{pageof@\thenotnumberedsection}\endcsname
        \show\scr@subsubsection@sectionindent
      }%
    }%
  }%
}{}{}

This would result in a last page:

but looks very strange in most cases, e.g., page 58 of the example:

If would recommend to not use this! If could be better to number the \subsubsection using

\setcounter{secnumdepth}{\subsubsectionnumdepth}