\leqno is broken in r2l paragraphs.

[Udi-Fogiel]

This is more a bug in the LuaTeX engine, but babel need to be aware of that (and maybe to find a workaround if it won't be fixed).
Here is a small example

\pardirection=1
$$1+2=3 \leqno(1)$$
\bye

This break files that use r2l script as a second language.
Here is an example using babel

\documentclass{article}

\usepackage[hebrew,english,provide*=*,bidi=basic]{babel}

\begin{document}
\selectlanguage{hebrew}
\begin{equation}
1+2=3
\end{equation}
\end{document}

I've sent an email to luatex-dev, I'll update if and when I'll get an answer.

[jbezos]

This wouldn’t be the first workaround to fix some issue in luatex math. Currently there is a dirty hack related to \[ ... \] (search the dtx for the string ‘YA luatex bug?’), which seems to do the job and I haven't had time to investigate further.

[Udi-Fogiel]

The only idea I have for a fix is to use \eqno, fix the horizontal placing by translating the box, and then fix the vertical spacing with the aid of the pre_append_to_vlist_filter callback (which is not a part of the kernel yet I believe, the lua-ul package defines it in a lua module).

Do you think it is too involved for such a small thing?

[jbezos]

@Udi-Fogiel Any answer from luatex-dev? I've devised a pure TeX solution, somewhat ad hoc and hackish, which seems to work, at least in simple cases:

\AddBabelHook[hebrew]{equation}{afterextras}{%
  \def\beforeequation{%
    \vskip-\baselineskip\vskip\abovedisplayskip
    \begin{otherlanguage*}{english}\pardir TLT
    \def\afterequation{\end{otherlanguage*}}}}
    
\let\afterequation\relax
\AddBabelHook[english]{equation}{afterextras}{%
  \let\beforeequation\relax}
  
\AddToHook{env/equation/before}{\beforeequation}
\AddToHook{env/equation/after}{\afterequation}

Although there are some workarounds in babel for a few luatex ‘oddities’, they are more or less simple, and necessary even in very basic documents. I don’t think that’s also the case, so I would say this bug should be fixed at the engine level.