thinspace will stay at a break but not hairspace, is this intended ?

[jfbu]

it is a question really, because I didn't check Unicode specs, but github is lacking an interface for that,

THIN SPACE is defined as \leavevmode\,\linebreak[0], hence if a break occurs at the penalty, the kern \, will stay at end of line.
The break may also happen before the kern, then it will disappear at start of next line.

edit: correction, due to the penalty following the kern, no break can happen before the kern (I got confused: if the kern had been followed by some glue, a break could happen before the kern; but not with penalties). TeX will complain of an overfull horizontal box, the kern will hang their invisibly in the margin.

On the other hand the HAIR SPACE is simply a glue, hence will not stay at end of a line. Is it deliberate that the two differ ? (this example is not convincing see next posts)

\documentclass{article}
\begin{document}

\begin{minipage}{\dimexpr 100pt+.16667em\relax}
\rule{100pt}{1pt}\,\linebreak[0]a

\rule{\linewidth}{1pt}

X\hrulefill X

\rule{\linewidth}{1pt}\rlap{\smash{\rule{0.1pt}{5\baselineskip}}}
\end{minipage}
\end{document}

[jfbu]

Perhaps this is more relevant, produces the same image and shows the phenomenon of the kern staying at end of line.

\begin{minipage}{\dimexpr 100pt+.16667em\relax}
\hrulefill\,\linebreak[0]

a

\rule{\linewidth}{1pt}

X\hrulefill X

\rule{\linewidth}{1pt}\rlap{\smash{\rule{0.1pt}{5\baselineskip}}}

\end{minipage}

[jfbu]

ok, perhaps a more convincing example

\documentclass{article}
\begin{document}

\begin{minipage}{\dimexpr 100pt+.16667em\relax}
\rule{45pt}{1pt}\ \ \rule{45.2pt}{1pt}x\,\linebreak[0]y

\rule{\linewidth}{1pt}\rlap{\smash{\rule{0.1pt}{2\baselineskip}}}
\end{minipage}
\end{document}

and adding \showoutput and examining the log one sees the kern refused to disappear in a break. The space at middle of line was shrinked to allow the kern to stay without creating overfull box.

.....\hbox(4.30554+0.0)x101.66672, glue set - 0.965  <=== compressed glue
......\hbox(0.0+0.0)x0.0
......\hbox(1.0+0.0)x45.0
.......\rule(1.0+0.0)x45.0
......\glue 3.33333 plus 1.66666 minus 1.11111
......\glue 3.33333 plus 1.66666 minus 1.11111
......\hbox(1.0+0.0)x45.2
.......\rule(1.0+0.0)x45.2
......\OT1/cmr/m/n/10 x
......\kern 1.66672 <=== kern is here at end of line
......\penalty 0
......\glue(\rightskip) 0.0

[wilx]

Thank you for the bug report. I have pushed a fix for this that uses \newunicodechar{ }{\leavevmode\linebreak[0]\,} for THIN SPACE instead.

[jfbu]

and what about FIGURE SPACE and PUNCTUATION SPACE ? the \hphantom creates boxes which do not allow breaks. Again, I know nothing about how Unicode defines the behaviour of those at end of lines... but perhaps a \linebreak[0] could be added too after the \leavevmode.

[jfbu]

about my last comment, perhaps wrong proposal: the box will be there at start of new line. For better uniformity with THIN SPACE the code would have to create a \kern, not a box. Hence, not use \hphantom. But again, perhaps a FIGURE SPACE must always have in all circumstances the width of a figure, then having such a space at start of next line after a break is ok. No idea what's the thing to do.

[wilx]

and what about FIGURE SPACE and PUNCTUATION SPACE ? the \hphantom creates boxes which do not allow breaks. Again, I know nothing about how Unicode defines the behaviour of those at end of lines... but perhaps a \linebreak[0] could be added too after the \leavevmode.

According to Whitespace character - Wikipedia, FIGURE SPACE is not breakable space. However, I think you have a point with PUNCTUATION SPACE.

[wilx]

I have pushed 22f9ec7 to fix the issue with PUNCTUATION SPACE.

[jfbu]

This whole matter is complicated because for some reason LaTeX implemented \, as \kern. But for example the \hspace ultimately does a \hskip. If we put \leavevmode[0] after an \hspace it is like \allowbreak\hspace{...}. But after a \kern there is no permutation hence it remains like \kern <dimen>\allowbreak and not \allowbreak\kern<dimen>.

Notice that Plain TeX has a \, only for math mode. The \, of LaTeX works both in text and math mode. It does a \thinspace in text mode, but this \thinspace is \kern .16667em.

[wilx]

So I have made the change to use \allowbreak.

[wilx]

I think this is fixed now.