Make section unaffected by parfillskip

Question

To avoid widow words at the last line of a paragraph, I adjust parfillskip:

parfillskip 0pt plus 0.75textwidth

While this gives nicer paragraphs, it also stretches section titles a little, as you can see below.
The first title is with the default parfillskip, the second with 0pt plus 0.75textwidth:

How can I keep my parfillskip setting for paragraphs, but not for titles?

This code showcases the effect for text and titles (preview):

documentclass{article}
usepackage{lipsum}
begin{document}
section{This is a section title}
lipsum[1]
section{This is a section title}

% Fill the last line of paragraphs for minimum 25%
parfillskip 0pt plus 0.75textwidth
section{This is a section title}
lipsum[1]
end{document}

egreg · Accepted Answer

The section title is typeset by @sect, which happily encloses the relevant part in a group. At the end it adds @@par, which is the primitive par.
Thus we can safely patch @sect (for numbered sections) and @ssect (for unnumbered sections):
documentclass{article}

usepackage{etoolbox}
makeatletter
patchcmd{@sect}{begingroup}{begingroupparfillskip=0pt plus 1filrelax}{}{}
patchcmd{@ssect}{begingroup}{begingroupparfillskip=0pt plus 1filrelax}{}{}
makeatother

usepackage{lipsum}
begin{document}
section{This is a section title}
lipsum[1]

% Fill the last line of paragraphs for minimum 25%
parfillskip 0pt plus 0.75textwidth
section{This is a section title}
lipsum[1]
end{document}

Ruben Verborgh · Answer

For the memoir class, it suffices to add to the preamble:

setsecheadstyle{Largeparfillskip=0pt plus 1fil}

Donald Arseneau · Answer

Since section titles should not be formatted with flush-right margins in any case -- despite the default document classes doing exactly that -- you should declare raggedright in the definition of the sectioning; and once you do that, the parfillskip is irrelevant.
Define the sectioning however you want:

with memoir or koma or other nice document class;
using a package like titlesec or some such;
with sectioning of the standard document classes using @startsection
or, the most hackish, patch commands like @sect

For example, the basic "article" class uses
newcommandsection{@startsection {section}{1}{z@}%
                               {-3.5ex @plus -1ex @minus -.2ex}%
                               {2.3ex @plus.2ex}%
                               {normalfontLargebfseries}}

so a document setup could include a renewcommand of that with raggedright inserted in the font-style argument
makeatletter
renewcommandsection{@startsection {section}{1}{z@}%
                               {-3.5ex @plus -1ex @minus -.2ex}%
                               {2.3ex @plus.2ex}%
                               {normalfontraggedrightLargebfseries}}

Make section unaffected by parfillskip

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