A hyphen followed by a word fails to protrude the left margin in LuaTeX.

[Witiko]

Description

A hyphen followed by a word fails to protrude the left margin in LuaLaTeX with current TeX Live 2025.

Minimal example demonstrating the issue

\documentclass{article}
\usepackage{microtype}
\begin{document}
\noindent
\texttt{-} \\
\texttt{-\kern0pt foo} \\
\texttt{-foo}
\end{document}

[schlcht]

This is the engine's behaviour, and there is nothing microtype can do. The following plain luatex file will exhibit the same quirk:

\protrudechars2
\lpcode\font`-=500
% OR: 
%\input luaotfload.sty
%\directlua{fonts.protrusions.setups["test"] = {[0x002D] = { 1, 1 }}}
%\font\x=LatinModernRoman:protrusion=test; \x
\noindent
-\hfil\break
-foo\hfil\break
-\kern0pt foo
\bye

I've just tested with some older versions, and it's been like this since TL 2018, while luatex 1.0.4 from TL 2017 still yielded the expected protrusion. You might want to report this to the luatex folks?

[Witiko]

You might want to report this to the luatex folks?

Not really! I can forward your MWE to the LuaTeX mailing list, but a report coming straight from you would probably be more useful to them.

[Witiko]

I forwarded your MWE to TUG's LuaTeX mailing list with a backlink to this issue ticket.

[Witiko]

@schlcht According to @hanshagen's response, the issue seems to be due to \automatichyphenmode, which is set to 0 by default in LuaTeX and which turns some hyphens into discretionary nodes. The solution is to use \automatichyphenmode = 1, which uses a different algo that treats leading hyphens better, or \automatichyphenmode = 2, which disables this behavior.

While this indeed fixes both your plain TeX example from #61 (comment) and my LaTeX example from the original post of this issue ticket, it still doesn't work for the actual article (DOI 10.5300/2025-1-4/11, page 13), where removing \kern0pt in the source file (mirror) still prevents the protrusion of the text "-lj index" at the beginning of a line, regardless of whether \automatichyphenmode has been switched beforehand. However, I was unable to isolate a small MWE yet.

[schlcht]

Good to know that there's a remedy! Please do follow-up if you're able to isolate a mwe.