As an aside before I start work on belatedly fixing some of
unicode-math's more pressing shortcomings, I've also started work tidying up the
breqn package: https://github.com/wspr/breqn
breqn package was the brainchild of Michael J. Downes (of
amsmath fame), and later updated by Morten Høgholm as part of his honours thesis. The package is explained in Michael’s 1997 TUGboat article ‘Breaking Equations’, and Morten’s thesis is available online if you know where to look (I don’t know how long it will be available there).
Since I have so much trouble keeping up with LaTeX work as it is, why would I take another package under my belt? In short, partly because
breqn vastly alters the implementation of LaTeX mathematics, so I need to eventually know how to hook into it to properly support
unicode-math. But also partly because of its heritage; such a promising piece of work shouldn’t be neglected—I used
breqn in my own PhD thesis, and while I could have done without it, once you start with
breqn it is hard to go back to
If you’re interested, I need help generating test files; once they’re sufficiently comprehensive I’ll begin the process of translating the code into LaTeX3. Hopefully I can fix a bug or two along the way.