User-defined marks swallowed if inside multicols

[cspiel]

Brief outline of the bug

I find that even with the latest (released) multicol.sty, v1.9h,
user-defined marks are recorded as proven by the log file but
they do not quite make it to \FirstMark or \LastMark if the
insertion points are within a multicols environment.

Otherwise the \NewMarkClass mechanism works flawlessly.

In the example given below I'd expect the head to show a-n,
however what I get is a-b.

The problem may share a distant relationship with issue #1130.

Minimal example showing the bug

\RequirePackage{latexbug}
\documentclass{article}

\usepackage{multicol}

\NewMarkClass{DictMark}
\DebugMarksOn

\makeatletter
\def\@oddhead{\FirstMark{DictMark}--\LastMark{DictMark}\hfill\thepage}
\let\@evenhead=\@oddhead
\makeatother

\begin{document}
\InsertMark{DictMark}{a}a
\InsertMark{DictMark}{b}b

\begin{multicols}{2}
\InsertMark{DictMark}{m}m
\InsertMark{DictMark}{n}n
\end{multicols}

%%--\InsertMark{DictMark}{z}z
\end{document}

Log file (required) and possibly PDF file

l3marks.log
l3marks.pdf

[FrankMittelbach]

The problem is that multicol is still not yet enabled to support the new mark mechanism. I'm aware of that but haven't found the time to address this.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity.

[FrankMittelbach]

Support for the new mark mechanism has now been added to multicol. At the moment it is still in its own branch but will move soon to the develop branch and then show up in the next dev release and eventually in the June 2025 release of LaTeX, so this is just a heads-up that something is actually moving. Feedback on the code and the documentation welcome:
see pull request #1548

I should say, however, that it may not be easy to run the code in the branch as it is based on code changes in ltmarks.dtx, which means one has to build a private format to run it (at the moment), i.e., the multicol.dtx is not enough.

[cspiel]

@FrankMittelbach: THX for the tedious work and extra thanks
for warning me of the additional dependencies still involved.

I'll test your new code as soon as multicol.dtx will be enough.