support footnotes also if tagging is active

[hpvd]

Description

support footnotes also if tagging is active
currently there is a warning:

Package microtype Warning: Unable to apply patch footnote' on input line 21.`

Minimal example demonstrating the issue

update code to current tagging best practice
https://latex3.github.io/tagging-project/documentation/prototype-usage-instructions.html
plus added title for passing pdf validation
on https://dev.verapdf-rest.duallab.com/

original: latex3/tagging-project#67

\DocumentMetadata{
  lang        = de,
  pdfversion  = 2.0,
  pdfstandard = ua-2,
  pdfstandard = a-4f, %or a-4
  testphase   = 
   {phase-III,
    title,
    table,
    math,
    firstaid}  
}

\documentclass{article}

\usepackage{microtype}

\title{test title}

\begin{document}

Simple text

\end{document}

[schlcht]

Thanks for the report. I was a bit hesitant as yet, thinking that tagging was still somewhat of a moving target. But let's see...

[u-fischer]

@schlcht regarding your comment in the commit

% \item The new footnote code (in \file{latex-lab-footnotes.ltx}), which is meant
% to facilitate tagging, introduces many hooks, among them |fntext/begin|, which
% would seem appropriate for us. Unfortunately, however, we cannot use it, as we'd
% stumble over the hook management itself.

and your following patch: could you open an issue at https://github.com/latex3/tagging-project with an example why the hook doesn't work for you? I mean the main point of the new code and all the hooks is to avoid that people have to patch around and if that this doesn't work or is insufficient, we should correct the code.

[schlcht]

@u-fischer The problem is that if I insert \leftprotrusion via the hook management, this command (which peeks ahead) would see the two hook commands \__hook_toplevel fntext/begin and \__hook_next fntext/begin instead of the actual contents of the footnote. To stick with the hook interface, I see two potential solutions: (1) either make microtype aware of these internal commands, so that it would peek ahead, or (2) add something like \@expandtwoargs\leftprotrusion to the hook. While neither of these solutions will actually patch internal code, both of them would still rely on the internal code structure (i.e. the fact that there are exactly two commands following, or their names). So I'm not quite sure whether either of these solutions would really be that much cleaner.

BTW: Is it within the specs of fntext/begin to contain actual text to be typeset?

[u-fischer]

@schlcht Sorry I was a bit unclear. I quite understand (and expected) that it doesn't work in a hook. But imho using a hook for such peek ahead code isn't the right place anyway. A hook can be used by other packages and you do not have full control about when your chunk is executed. From your patch it looks as if \ignorespaces#1 should be something like \socket_use:n{fntext/peek}{#1} where the default plug simply adds the \ignorespaces. Then you could exchange the plug. So make a feature request about your use case!

[schlcht]

@u-fischer ah, yes, that makes sense. I'll file a request later...

[schlcht]

Just FYI: I've left the fix included in the latest release even if @u-fischer isn't happy with it (as am I). Therefore this issue should remain open until microtype can use the relevant socket (once it's been put into place by the latex team).