[luatex] math display box structure

[David Carlisle]

Hi,

luatex constructs boxes for display math differently to classic tex,
typically adding kerns rather than shifting.

In simple cases it makes the same visual output but it is easy to find
cases where the typeset output is different


\setbox0\vbox{$$ x \eqno (1)$$
\par
\unskip
\unpenalty
\global\setbox1\lastbox}

\the\wd1

\box1

\bye


However even when the typeset output looks the same the fact that the
box log is so different makes regression testing quite hard. The
recent bug reports about \choose
and \thinmuskip only came to light after manually filtering out
hundreds of lines of box differences due to the different box
constructs.

Accepted that luatex logging is never exactly the same as pdftex's, is
there a particular new functionality that is gained by these changes,
or failing that is there any documentation (other than the luatex
source code) of the expected places where the box structure for
mathematics will be different, so that we can try to automate
filtering out these differences for regression testing?

David