Jan Braun <Jan.Braun@klein-gallien.org> writes:

>> But even with this code added: we may have to dig deeper 
>> into the problem?

I think I had a typo in the code I suggested, it should read:

(with-eval-after-load "tex-mode"
  (add-hook 'doctex-mode-hook
            (lambda ()
              (setq-local latex-outline-regexp
                          (concat "[% \t]*" "\\\\"
                                  (regexp-opt
                                   (append latex-metasection-list
                                           (mapcar #'car latex-section-alist))
                                   t)))
	      (setq-local outline-regexp latex-outline-regexp))))

Now take a small example like this:

%\iffalse
%<*driver>
\documentclass{ltxdox}
\usepackage{microtype} % and other usual LaTeX preamble stuff
\EnableCrossrefs
\CodelineIndex
\RecordChanges
\setcounter{IndexColumns}{2}
\begin{document}
\DocInput{jb-test.dtx}
\end{document}
%</driver>
%\fi
%
% \section{Introduction}
% This is some introduction.
%
% \section{User macros}
% This is some text.
% And this is some text
%
% \section{Implementation}
%
%    \begin{macrocode}
%<*package>
%    \end{macrocode}
%
%    \begin{macrocode}
\def\foo#1{%
  $#1$%
}
%    \end{macrocode}
%
%    \begin{macrocode}
%</package>
%    \end{macrocode}
%
%\endinput
% Local Variables:
% mode: doctex
% End:

When I start Emacs with 'emacs -Q', open the file above, do
'M-x outline-minor-mode RET' followed by 'C-c @ C-t', this is what I
see: