Changes in horizontal spacing in some equations

[jovyntls]

Brief outline of the bug

The spacing of characters in some equations has changed from the TeXLive 2022 distribution to TeXLive 2023 distribution.
It seems to occur for equations using ~ with an operator like - or \leq.

Minimal example showing the bug

\RequirePackage{latexbug}
\documentclass{article}

\begin{document}

	\begin{equation}
	  -1,~-2
	\end{equation}
  
\end{document}

Log file (required) and possibly PDF file

main.log

PDF file from TeXLive 2022:
main2022.pdf

PDF file from TeXLive 2023:
main2023.pdf

Using the diff-pdf tool to superimpose both PDF files shows the difference in alignment (red is 2022, blue is 2023):
outdiff.pdf

[muzimuzhi]

In LaTeX2e 2023-11-01 (see e98027d), to make it safe in csnames, definition of active tilde was changed from

\def~{\nobreakspace{}}

to

latex2e/base/ltspace.dtx

Lines 1229 to 1234 in 4472861

	\protected\edef~{%
	\noexpand\ifincsname\noexpand\expandafter\string~%
	\noexpand\else
	\noexpand\expandafter\noexpand\nobreakspace
	\noexpand\fi
	}

The new definition acts like \nobreakspace without trailing pair of braces when used in non-csnames, for example in math equations.

There's even a code comment (added by a4df9c4) which explains why the {} is now unneeded.

latex2e/base/ltspace.dtx

Lines 1217 to 1220 in 4472861

	% The fragile version of |~| needs a brace group after \cs{nobreakspace}
	% to prevent loss of spaces if it occurs in an expansion context. That's
	% not an issue with the updated \cs{protected} definition, so we keep the
	% code shorter and avoid that.

I think current behavior of ~ in math is more correct. Note the spacing (actually the math class of minus sign) between minus sign and number 2 was interfered by the trailing {} in ~ prior to LaTeX2e 2023-11-01.

Also maybe math spacing commands \,, \:, and \; are more recommended than ~.

\RequirePackage{latexbug}
\documentclass{article}

\begin{document}
Before \LaTeXe{} 2023-11-01
\begingroup
\def~{\nobreakspace{}}
\begin{equation}
  -1,~ -2
\end{equation}
\endgroup

Starting from \LaTeXe{} 2023-11-01
\begin{equation}
  -1,~ -2
\end{equation}

[jovyntls]

I see, thank you so much for the detailed explanation! Should I close this issue?

[FrankMittelbach]

I would say so, but not the other one you opened up in colortbl because while releated that nevertheless might need some actions.