wrong alignment of multline+fleqn if math direction differs from the vertical list direction

[Udi-Fogiel]

\documentclass[fleqn]{article}

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

\begin{document}
\begin{equation}
a+b=c
\end{equation}
\begin{multline}
a+b+c+d+e+f+g+h+i+j+k+\\
l+m+n+o+p+q+r+s+t+w+x+y+z
\end{multline}

\selectlanguage{english}
\begin{equation}
a+b=c
\end{equation}
\begin{multline}
a+b+c+d+e+f+g+h+i+j+k+\\
l+m+n+o+p+q+r+s+t+w+x+y+z
\end{multline}
\end{document}

[Udi-Fogiel]

Actually the multline is aligned okay, but it is making the whole page box bigger (why? seems odd choice to make them wider than \textsize), which makes the alignment of all other lines visible

With TLT vertical list:

With TRT vertical list:

The multline position is not changing because they fit tightly in the page.

[jbezos]

@Udi-Fogiel The box is bigger even with pdflatex and without babel. I’m trying to understand the logic of multline, as well as how it deals with \@mathmargin. Enlarging the box seems, indeed, somewhat odd.

[Udi-Fogiel]

@jbezos I would also check the logic of \kern-\alignsep@ in \math@cr@@@align: latex3/latex2e#1960

[jbezos]

Here is an example with vanilla pdflatex. Although \hrule is not strictly a latex command, it’s documented and used in TLC.

\documentclass[fleqn]{article}

\usepackage{amsmath}
\usepackage{showframe}

\begin{document}

\begin{multline}
a+b+c+d+e+f+g+h+i+j+k+\\
l+m+n+o+p+q+r+s+t+w+x+y+z
\end{multline}

\hrule

\end{document}

[Udi-Fogiel]

Ah, good to know it has other implications. So should be fixed elsewhere I guess?

[jbezos]

Before definitively blaming amsmath, I’d like to know exactly what's going on and find a workaround.

[Udi-Fogiel]

Opening a bug report elsewhere or inviting people so more people can help does not mean it is definitely the fault of amsmath. I'm not sure who was/is actively involved with amsmath, but having an explanation from them will probably be easier/faster.

[Udi-Fogiel]

@jbezos

\documentclass[fleqn]{article}

\usepackage{amsmath}
\usepackage[english,hebrew,provide*=*,bidi=basic]{babel}
\makeatletter
\bbl@sreplace\multline@\crcr{\if@fleqn\tabskip\z@skip\fi\crcr}

\begin{document}
\begin{equation}
a+b=c
\end{equation}
\begin{multline}
a+b+c+d+e+f+g+h+i+j+k+\\
l+m+n+o+p+q+r+s+t+w+x+y+z
\end{multline}

\selectlanguage{english}
\begin{equation}
a+b=c
\end{equation}
\begin{multline}
a+b+c+d+e+f+g+h+i+j+k+\\
l+m+n+o+p+q+r+s+t+w+x+y+z
\end{multline}
\end{document}

Or with your example

\documentclass[fleqn]{article}

\usepackage{amsmath,showframe,xpatch}
\makeatletter
\xpatchcmd{\multline@}{\crcr}{\if@fleqn\tabskip\z@skip\fi\crcr}{}{}

\begin{document}

\begin{multline}
a+b+c+d+e+f+g+h+i+j+k+\\
l+m+n+o+p+q+r+s+t+w+x+y+z
\end{multline}

\hrule

\end{document}