Interaction between subalign and unicode-math

Question

For many years I've been using the subalign environment described in Align in Substack. However, with a fully updated version of TexLive 2023 I've recently observed a negative interaction with unicode-math. Consider the following MWE:

\documentclass{article}
\usepackage{fontspec}
\usepackage{unicode-math}
\usepackage{amsmath}
\makeatletter
\newcommand{\subalign}[1]{%
  \vcenter{%
    \Let@ \restore@math@cr \default@tag
    \baselineskip\fontdimen10 \scriptfont\tw@
    \advance\baselineskip\fontdimen12 \scriptfont\tw@
    \lineskip\thr@@\fontdimen8 \scriptfont\thr@@
    \lineskiplimit\lineskip
    \ialign{\hfil$\m@th\scriptstyle##$&$\m@th\scriptstyle{}##$\hfil\crcr
      #1\crcr
    }%
  }%
}
\makeatother
\begin{document}
  [
  \sum_{\substack{n=1\n\neq i}}^N
  \sum_{\subalign{n&=1\n&\neq i}}^N
  \sum_{\subalign{uvw&=1\n&\neq ijk}}^N
  ]
\end{document}

which renders as:

with there being a large amount of extra vertical spacing in the subalign environment but not in the substack environment. Commenting the unicode-math use resolve these issues.

Answer 1

LuaTeX has a completely different method for math font parameters.

Here's a fixed version that produces (essentially) the same result with pdflatex, xelatex and lualatex.

\documentclass{article}
\usepackage{iftex}
\iftutex % here just to be able to compare pdflatex and lualatex
\usepackage{fontspec}
\usepackage{unicode-math}
\fi
\usepackage{amsmath}
\makeatletter
\newcommand{\subalign}[1]{%
  \vcenter{%
    \Let@ \restore@math@cr \default@tag
    \ifluatex
      \baselineskip\Umathstacknumup\scriptstyle
      \advance\baselineskip\Umathfractiondenomdown\scriptstyle
      \lineskip=3\Umathfractiondenomvgap\scriptstyle
    \else
      \baselineskip\fontdimen10 \scriptfont\tw@
      \advance\baselineskip\fontdimen12 \scriptfont\tw@
      \ifxetex\baselineskip=0.724\baselineskip\fi
      \lineskip\thr@@\fontdimen8 \scriptfont\thr@@
    \fi
    \lineskiplimit\lineskip
    \ialign{\hfil$\m@th\scriptstyle##$&$\m@th\scriptstyle{}##$\hfil\crcr
      #1\crcr
    }%
  }%
}
\makeatother
\begin{document}
  [
  \sum_{\substack{n=1\n\neq i}}^N
  \sum_{\subalign{n&=1\n&\neq i}}^N
  \sum_{\subalign{uvw&=1\n&\neq ijk}}^N
  ]
\end{document}

output with pdflatex

output with lualatex