I think any build errors are from environment issues rather than my change, which is ridiculously small (one line). @alerque would you be able to take a look at this? This was breaking a lot of my documents (which use some of Knuth's glue-tricks), so I think others would be also interested in fixing this edge case.

which use some of Knuth's glue-tricks

Hi @holdenrohrer
Can you possibly develop further which kind of tricks, possibly with a MWE?
(I am experimenting other page building strategies and I am interested in knowing which differences with the TeX-derived one I can also face there).

I think any build errors are from environment issues rather than my change

Yep, automated CI test are currently broken for another reason (#2297).

Sure, here's a simple example using vfill:

%SILE
\begin{document}
\use[module=packages.rules]

\define[command=segment]{%
  \hrule[height=2in]
  \vfill
}

\segment
\segment
\segment
\segment
\segment
\segment
\segment
\segment

\end{document}

%TeX
\def\segment{%
  \vrule height 2in
  \vfill
}
\segment
\segment
\segment
\segment
\segment
\segment
\segment
\segment
\bye

The TeX document generates 2 pages, with 4 rules on each page.
The SILE document generates 5 pages, with one rule on the first 4 pages and 4 on the last. (I'm using letterpaper)

This means that SILE is packing content really inefficiently when there are a bunch of glues on the page.

This is a very simple version of a trick that Knuth mentions on page 111 of the TeXbook:

The most interesting macro that plain TEX provides for page make-up is called
\filbreak. It means, roughly, “Break the page here and fill the bottom with
blank space, unless there is room for more copy that is itself followed by \filbreak.”
Thus if you put \filbreak at the end of every paragraph, and if your paragraphs aren’t
too long, every page break will occur between paragraphs, and TEX will fit as many
paragraphs as possible on each page. The precise meaning of \filbreak is

\vfil\penalty-200\vfilneg

according to Appendix B; and this simple combination of TEX’s primitives produces
the desired result: If a break is taken at the \penalty-200, the preceding \vfil will
fill the bottom of the page with blank space, and the \vfilneg will be discarded after
the break; but if no break is taken at the penalty, the \vfil and \vfilneg will cancel
each other and have no effect.

As it is, this macro couldn't work in SILE because SILE tries to break too early rather than pack more material into the page. However, I'm pretty sure with my change and a way to discard glue at the top of the page (which might already be the default iirc?), these macros would work.