amsart figure captions with TeXlive 2021

Question

I am finding misaligned figure captions when using TeXlive 2021 with amsart. A test file is here:

\documentclass{amsart}
\begin{document}

\hbox to\hsize{X\hfil \TeX{Live} 2021 on Mac\hfil X}

\begin{figure}[h]
\hbox to\hsize{X\hfil X\hfil X}
\caption{XXXXX}
\end{figure}
\end{document}

The first hbox is just to show the margins. Running with TeXlive 2021 (June 2021) on Macosx 11.6, the caption is not centered:

However, running with TeXlive 2019 on Ubuntu 20.04, the centering is correct: I have checked that the part of amsart.cls concerned with figure captions has not changed between these versions. Also, it doesn't make any difference whether latex or pdflatex is used. The problem only occurs for captions that fit on one line.

These are the only two versions of TeXlive that I have easy access to.

Answer 1

The original file works without error in LaTeX releases from

LaTeX2e <2021-11-15> pre-release-2 (develop 2021-9-20 branch)

---

Original answer

The class needs to be updated for the 2021 latex release, an extra unkern is needed

update actually we will probably adjust things in the latex format so the following patch will not be necessary after the next latex release https://github.com/latex3/latex2e/pull/671

\documentclass{amsart}
\makeatletter
\long\def\@makecaption#1#2{%
  \setbox\@tempboxa\vbox{\color@setgroup
    \advance\hsize-2\captionindent\noindent
    \@captionfont\@captionheadfont#1\@xp\@ifnotempty\@xp
        {\@cdr#2\@nil}{.\@captionfont\upshape\enspace#2}%
    \unskip\kern-2\captionindent\par
    \global\setbox\@ne\lastbox\color@endgroup}%
  \ifhbox\@ne % the normal case
    \setbox\@ne\hbox{\unhbox\@ne\unskip\unskip\unpenalty\unkern\unkern}%
                                                               %%%%%%% extra unkern
  \fi
  \ifdim\wd\@tempboxa=\z@ % this means caption will fit on one line
    \setbox\@ne\hbox to\columnwidth{\hss\kern-2\captionindent\box\@ne\hss}%
  \else % tempboxa contained more than one line
    \setbox\@ne\vbox{\unvbox\@tempboxa\parskip\z@skip
        \noindent\unhbox\@ne\advance\hsize-2\captionindent\par}%
  \fi
  \ifnum\@tempcnta<64 % if the float IS a figure...
    \addvspace\abovecaptionskip
    \hbox to\hsize{\kern\captionindent\box\@ne\hss}%
  \else % if the float IS NOT a figure...
    \hbox to\hsize{\kern\captionindent\box\@ne\hss}%
    \nobreak
    \vskip\belowcaptionskip
  \fi
\relax
}
\makeatother
\begin{document}

\hbox to\hsize{X\hfil \TeX{Live} 2021 on Mac\hfil X}

\begin{figure}[h]
\hbox to\hsize{X\hfil X\hfil X}
\caption{XXXXX}
\end{figure}
\end{document}