AlbertNashr
Continuing http://topanswers.xyz/tex?q=3768 , for all the theorem-like environments in our document, we wish to have a little bit more horizontal space (than the default space) separating the head of the environment (whether it is fully automatically generated or also contains a user-given part) from its contents. To this end, we tried to redefine \@thmcounterend (thanks to [@DavidCarlisle][1] and [@Skillmon][2]):

\documentclass[twocolumn]{svmono}% v5.10 (2021/09/08) from http://www.springer.com/gp/authors-editors/book-authors-editors/your-publication-journey/manuscript-preparation#toc-49268 or directly from http://resource-cms.springernature.com/springer-cms/rest/v1/content/20566/data/monographs
\overfullrule=1mm
\pagestyle{empty}
\newcommand{\test}[1]{
\noindent Remark with a short name:
\begin{remark}[Short name]
Body of a remark with a short name.
\end{remark}
Remark without a special name:
\begin{remark}
Body of an unnamed remark.
\end{remark}
Remark with a line-filling, long name and a label:
\begin{remark}[Some long remark name.]\label{#1}
Body of a remark with a long name.
\end{remark}
}%
\begin{document}%
\section*{Originally}
\test{RemarkLabelOne}
\newpage
\section*{Redefining \texttt{\textbackslash @thmcounterend}}
\makeatletter\def\@thmcounterend{\@ifnextchar\ {\hspace{2\fontdimen2\font plus2\fontdimen3\font minus2\fontdimen4\font}\@gobble}{ }}\makeatother
\test{RemarkLabelTwo}
\end{document}

Running pdflatex on this results in

[![output side-by-side][3]][3]

As we see, the original version is simply ugly: the title and the contents are separated by a too small space (cf. Remark 2). If we redefine \@thmcounterend to be a space (\def\@thmcounterend{ }  or \def\@thmcounterend{\ } or \def\@thmcounterend{\space}), an unwanted empty line after a line-long remark title may emerge.  If we redefine \@thmcounterend to redefine the space to be of the double length (\def\@thmcounterend{\def\ {\hspace{2\fontdimen2\font plus2\fontdimen3\font minus2\fontdimen4\font}}}), but the user does not supply a name, then the horizontal space after a numbered unnamed remark is still small as originally.  If we redefine \@thmcounterend to choose between the two options above (\def\@thmcounterend{\@ifnextchar\ {\hspace{2\fontdimen2\font plus2\fontdimen3\font minus2\fontdimen4\font}\@gobble}{ }}), we get a small but unwanted vertical space after a line-long named-remark title as in Remark 6.

How to globally change the definitions of the involved macros and environments (while preserving the LaTeX-document contents) so that more horizontal space is inserted between the end of a remark title and the beginning of a remark body whenever they are on the same line (and introduce no extra space if they are on separate lines)?

I cannot help thinking that redefining some other macro than \@thmcounterend might work better but don't have enough knowledge to find such a macro and change it myself.

Crossposts: http://latex.org/forum/viewtopic.php?f=4&t=35426&p=118775 , https://texwelt.de/fragen/29112 , http://golatex.de/viewtopic.php?f=4&t=25009 , and http://tex.stackexchange.com/questions/678941 .

[1]: http://tex.stackexchange.com/a/678928
[3]: https://i.stack.imgur.com/VKCni.png