[pretest] \pdfinterwordspaceon breaks math typesetting

[Ulrike Fischer]

The new handling of \pdfinterwordspaceon breaks the math with pdftex
in texlive 2025:

\ExplSyntaxOn\sys_ensure_backend:\pdf_uncompress:\ExplSyntaxOff

\documentclass{article}

\begin{document}
\pdfinterwordspaceon
$ a b \mathbf{u} $ $ a\mathbf{u} $ $ a\mathbf{b} $

\end{document}

When I compile this the first equation is correct. But the second is
missing the bold u, and the third uses an mathitalic b instead of a
bold b.

In the PDF one can see that in the follow-up equations the font is
no longer switched. Also the PDF shows real space chars inside the
math which looks wrong, as they don't make sense there: (I added
line breaks for better readability):

/F31 9.9626 Tf 148.712 707.125 Td [(a)]TJ/F40 9.9626 Tf( )Tj/F31
9.9626 Tf 5.266 0 Td [(b)]TJ /F37 9.9626 Tf [(u)]TJ

/F40 9.9626 Tf( )Tj

/F31 9.9626 Tf 13.962 0 Td [(au)]TJ

/F40 9.9626 Tf( )Tj

/F31 9.9626 Tf 14.952 0 Td [(ab)]TJ


In texlive 2024 the (correct) output looks like this:

/F31 9.9626 Tf 148.712 707.125 Td [(ab)]TJ/F37 9.9626 Tf 9.542 0 Td
[(u)]TJ

/F31 9.9626 Tf 9.686 0 Td [(a)]TJ/F37 9.9626 Tf 5.266 0 Td [(u)]TJ

/F31 9.9626 Tf 9.686 0 Td [(a)]TJ/F37 9.9626 Tf 5.266 0 Td [(b)]TJ


-- 
Ulrike Fischer 
http://www.troubleshooting-tex.de/