Recent package version leads to unexpected overful \hboxes

[mmaeusezahl]

Hi,

I hope that I'm not completely mistaken with this issue since I'm not a strong LaTeX package developer (more of a user), but I found the following behavior in a rather complex LaTeX document after updating TeXLive. I suspect that the origin comes from the microtype package.

MWE:

Essentially make a KOMA-Script headline and set the font to small caps.

\documentclass[a4paper]{scrbook} %
\overfullrule=1mm % just to show where the issue is
\RequirePackage[tracking=true]{microtype} %
\RequirePackage{scrlayer-scrpage}
\setkomafont{pageheadfoot}{\normalfont\scshape} %
\begin{document}
\chapter{Test}
\end{document}

I compile with LuaLaTeX. For reference, this is my latexmkrc:

#!/usr/bin/perl
$pdflatex = 'lualatex -file-line-error -synctex=0 -interaction=nonstopmode';
$pdf_mode = 1;
$out_dir = 'output/';

I do not expect an overfull hbox in this MWE, since the header will be empy.

Issue

Until a recent update this compiles without any issues and overful hboxes (the last working version that was shipped with nixpkgs and is therefore easy to test from my side was v3.1b).

After a recent update which introduced v3.2a I get the following hint for each line on each page in the header and footer (so already four times in this example):

Overfull \hbox (0.54749pt too wide) has occurred while \output is active
[]|[]| |
 []

Versions

Since I'm on NixOS I can inject older versions and could compare the exact same document once with microctype version v3.1b and once with v3.2a:

v3.1b.log
v3.2a.log

As far as I can see from the difference between the log files only the microtype package version changes. Anything else is pinned to the final texlive2024 release in nixpkgs. Some cache directories change, since they are only temporarily generated to have reproducible builds, but I would not assume that they are problem.

Therefore I suspect that there might be a bug in microtype?
Sadly my understanding of LaTeX package development goes only so far (and I hope that this is not some other package incompatibility) ^^

Thanks for looking into it!