Full line width content in flushed/center envs shows extra vertical spaces

SILE 0.14.8 (from the siletypesetter/sile:v0.14.8 docker image, to ensure my own build of current master wasn't causing the issue), but also reproduced with earlier versions (tested with siletypesetter/sile:v0.12.5)

Pretty minor issue, since there's a workaround anyway (see below), but I am not sure whether it's caused by some rounding issue or if there's something more tricky at play.

Scenario

The content in a "center", "raggedleft" or "raggedright" environment is exactly 100%lw. E.g. I observed it with a full-line table, but it would also be true with an image set at that width. In the MWE below I am just using rules to show the issue in a simple way.

\begin{document}
\neverindent
\nofolios
\use[module=packages.rules]
Justified 100\% content

\hrule[height=2pt, width=100%fw]\par
\hrule[height=2pt, width=100%fw]\par

Centered 100\% content = issue

\center{%
\hrule[height=2pt, width=100%fw]\par
\hrule[height=2pt, width=100%fw]\par
}

Centered 99.99999999999\% content

\center{%
\hrule[height=2pt, width=99.99999999999%fw]\par
\hrule[height=2pt, width=99.99999999999%fw]\par
}
\end{document}

Expected result

Vertical spacing isn't expected to differ from the justified case.

Observed result

Some glues or (zero?)boxes are pushed in their own line, leading to additional vertical spaces.

A workaround is to set the width to a smaller value. I was drastic above with the 99.9999...., even a mere 99.9% is usually sufficient and remains hardly visible.

sile-typesetter / sile

Full line width content in flushed/center envs shows extra vertical spaces #1757