Bug in 3.141592653-2.6-0.999995 (TeX Live 2023) with fontspec and tabularray?

[Karl Berry]

    That is impressive debugging indeed! How you (Ross) managed to
    identify that piece of code as the culprit is beyond me.

Agreed!

    It seems Don Knuth was a bit too optimistic when he wrote

As far as I can see, this issue has nothing to do with the tex.web
program that Knuth wrote.  D2Fix seems to be related mostly to reading
font dimensions from OpenType/TrueType fonts, which obviously original
TeX does not know anything about.

There are many reasons why line breaks might differ among engines when
it comes to features beyond original TeX -- fonts are the biggest, but
also microtype, language-specific features a la harfbuzz and icu, luatex
callbacks, and plenty more.

If one restricts oneself to using TFM-based fonts and no extensions, I
think it is still accurate that line breaks are the same among all
engines.

    Of course, such matter arises at the worst possible time, just as
    the compilation of tl2024 pretest is about to start.

Well, the worst possible time would be the day after the TL24 release :).
There's no technical or scheduling problem with making the change.  The
question is whether the change would simply shift the issue to a
different set of very-specific line widths, character sizes, etc., that
would have different breaks. That's my suspicion. I hope Jonathan K is
able to reply soon. --best, karl.