Fancyhdr's extra dot in header for section title: How to remove?

Question

I like to use Arabic numbers with a dot in front of section titles. So, for example 1. My section title instead of 1 My section title or something else. To achieve this, the syntactically simple option
renewcommandthesection{arabic{section}.}

works like a charm. The dot also appears in the table of contents, which is great. Next, I load the fancyhdr package (documentation). The aim is to obtain the identical format in fancyhdr's fancy header. By default (when renewcommandthesection{arabic{section}.} is in use), it gives two dots instead of the desired one dot, so 1.. My section title instead of 1. My section title. This is the only thing I would like to change.

After reading a little bit of the documentation, I guessed that
renewcommand{sectionmark}[1]{markright{thesection #1}}
would work. Compiling with Overleaf, this did not help. Compiling instead with my local (more updated) distribution the problem was solved.

Is this a reasonable/safe/canonical way to solve the problem?

("Safe" – likely to work in the future, not breaking something important in the background)

If not, what is a better/alternative solution? (Minimally invasive, syntactically simple solution is preferred)

MWE (uncomment for my attempted solution):
documentclass{book}

usepackage{fancyhdr}
usepackage{titlesec}
usepackage[colorlinks=true, linkcolor=blue, hypertexnames=false]{hyperref}

renewcommandthechapter{Roman{chapter}}
renewcommandthesection{arabic{section}.}
%renewcommand{sectionmark}[1]{markright{thesection #1}} <-- uncomment this for my attempt

begin{document}

clearpagethispagestyle{empty}
    {hypersetup{linkcolor=black}tableofcontents}
    pagestyle{fancy}
    thispagestyle{empty}
    
    chapter{My chapter title}
        section{My chapter title}
            pagebreak
            Why two dots in fancy header here?

end{document}

Output of page 5 (Overleaf, commented or uncommented – no difference):

Output of page 5 (local, commented):

Output of page 5 (local, uncommented):

I am running pdflatex 3.141592653-2.6-1.40.22 (MikTeX 21.3), and every package in my local distrubution was updated on 28/04/2021.

Werner · Accepted Answer

Without titlesec you have two things you need to update - the sectional unit number in the section title (@seccntformat) and what is printed in the ToC (cftsecafternsum from tocloft):

documentclass{book}

usepackage{fancyhdr}
usepackage{tocloft}

renewcommandthechapter{Roman{chapter}}
makeatletter
% Add dot after sectional unit number (for section, subsection, ...)
renewcommand{@seccntformat}[1]{csname the#1endcsname.quad}
makeatother
renewcommandthesection{arabic{section}}% Remove chapter number from section number
renewcommand{cftsecaftersnum}{.}% Add dot after section number in ToC

pagestyle{fancy}

begin{document}

tableofcontents

chapter{My chapter title}
section{My chapter title}
clearpage
Why two dots in fancy header here? No moreldots

end{document}

The reason for not including the period in thesection is because it would otherwise be included in refs to a section, which might be odd, as they can occur mid-sentence.

Simon Dispa · Answer

UPDATED You can use the titlesec package that you already loaded, to add the dot to the section number instead of redefining thesection.
In a similar way after the subsection number.
I added some dummy text to check the headers generated by fancyhdr.
Strictly speaking, fancyhdr is not needed to solve the issue of the double dot. But it gives you a better control of the headers and footers.
I added part of Werner`s code to add, for sections and subsections, the dot in the TOC (last figure).

% !TeX TS-program = pdflatex

documentclass{book}
usepackage{fancyhdr}

pagestyle{fancy}

usepackage[compact,explicit]{titlesec}

titleformat{section}{largebfseries}{}{0pt}{thesection .quad #1} % section number with dot
titleformat{subsection}{bfseries}{}{0pt}{thesubsection .quad #1} % subsection number with dot

usepackage[colorlinks=true, linkcolor=blue, hypertexnames=false]{hyperref} 
renewcommandthechapter{Roman{chapter}}

% *********************************** TOC
usepackage{tocloft} % added to correct TOC%
renewcommand{cftsecaftersnum}{.}% Add dot after section number in ToC
renewcommand{cftsubsecaftersnum}{.}% Add dot after subsection number in ToC
% *********************************** TOC

usepackage{kantlipsum}  % dummy text
begin{document}
    
    {hypersetup{linkcolor=black}tableofcontents}  
    clearpagethispagestyle{empty}
    
    chapter{My chapter title}  
    11. kant[11-15]
    
    section{My section title (sec)}
    12. kant[12-16]
    
        subsection{My subsection title (subsec)}
    14. kant[14-18]
    
    newpage
    Why two dots in fancy header here?

end{document}

Bernard · Answer

Since you load titlesec, you can use its titlelabel command:
documentclass{book}

usepackage{fancyhdr}
usepackage{titlesec}
usepackage[colorlinks=true, linkcolor=blue, hypertexnames=false]{hyperref}
titlelabel{thetitle.quad}
renewcommandthechapter{Roman{chapter}}

begin{document}

clearpagethispagestyle{empty}
    {hypersetup{linkcolor=black}tableofcontents}
    pagestyle{fancy}
    thispagestyle{empty}

chapter{My chapter title}
        section{My section title}
            pagebreak
            Why two dots in fancy header here?

end{document}

Fancyhdr's extra dot in header for section title: How to remove?

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