Overfull hbox in author list in acmart + texlive 2020

Question

There is a peculiar overfull hbox that seems to arise from the authornotemark in the following MWE:

documentclass[sigconf,nonacm]{acmart}
title{Test}
author{A}
authornotemark
affiliation{institution{T}country{R}}
author{B}
affiliation{institution{T}country{R}}

begin{document}
maketitle
end{document}

(pdf)latex reports an

Overfull hbox (4.18721pt too wide) in paragraph at lines 11–11

I have not found anything useful in the log regarding this. The overfull hbox happens when there is an authornotemark after the first of the two authors; it also occurs with more authors. Just adding a footnotemark in the author’s name is fine, though.
My local version is

This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020) (preloaded format=pdflatex)
LaTeX2e <2020-10-01> patch level 2
L3 programming layer <2020-10-27> xparse <2020-03-03>
Document Class: acmart 2020/11/15 v1.75 Typesetting articles for the Association for Computing Machinery
Document Class: amsart 2020/05/29 v2.20.6

No overfull hbox is reported on overleaf, with

This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020) (preloaded format=pdflatex 2020.9.10)  28 NOV 2020 01:21
LaTeX2e <2020-02-02> patch level 5
L3 programming layer <2020-07-17>
Document Class: acmart 2020/04/30 v1.71 Typesetting articles for the Association for Computing Machinery
Document Class: amsart 2020/05/29 v2.20.6

I have no idea how to troubleshoot this further, and I’m curious as to why this can happen.

acmart texlive 2020

David Carlisle · Accepted Answer

The difference is that the newest acmart.cls loads hyperxmp and that has some acmart-specific code to try to parse metadata from the class-specific title commands.
At one point the authors are set in a box and this results in the overfull box message but this box is just used for some internal checks and never output to the page, so the warning does not in fact relate to any over full output in the document.
It seems very dubious that the generic hyperxmp package is requiring acmart-specific code but the simplest fix is just to silence warnings while doing the trial typesetting
line 596 of hyperxmp.sty is
    setbox0=hbox{addresses}%

changing it to
   setbox0=hbox{hfuzzmaxdimenaddresses}%

silences the warning. You could raise this with the hyperxmp author.

Overfull hbox in author list in acmart + texlive 2020

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