\multline + fleqn result with a \halign wider than \displaysize

[Udi-Fogiel]

Brief outline of the bug

In the example below one can see that the page box is wider than \textsize=345pt.
In fact it is 370pt and so it is wider by 25pt, so it led me to believe there is an extra \tabskip
to the right. I think what happens is that \tabskip is set by \tabskip \if@fleqn \@mathmargin \else \z@skip \fi, but this skip is only needed on the left side.

A possible solution:

\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}

Minimal example showing the bug

Example by @jbezos

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass[fleqn]{article}

\usepackage{amsmath}

\begin{document}
\showoutput
\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}

Log file (required) and possibly PDF file

test.log

test.pdf

[Udi-Fogiel]

Actually without fleqn it is zero anyway, so maybe \xpatchcmd{\multline@}{\crcr}{\tabskip\z@skip\crcr}{}{} is sufficient.