Lualatex beamer breqn: Text appears four timestimestimestimes, why and how to mitigate?

Question

In the following example, why does the word Spam appear four times_{times_{times_times}}?

$ cat mwe.tex 
\documentclass{beamer}
\usepackage{breqn}
\newcommand{\spam}{\textrm{Spam}}
\begin{document}
\begin{frame}
\[ \spam \]
\end{frame}
\end{document}

...and what can I do to get it just once?

I'm using:

LuaTeX, Version beta-0.80.0 (TeX Live 2015) (rev 5238),
beamer 2015/01/05 3.36,
breqn 2015/08/11 v0.98d.

Can't reproduce. Which tex version are you using? I tried with TeXLive 206 — samcarter_is_at_topanswers.xyz, Jul 04 '16 at 14:04
@samcarter The problem is caused by compilation with LuaTeX, something around \mathchoice. — Heiko Oberdiek, Jul 04 '16 at 14:05
@UlrikeFischer Because of ignorance / old bad habits / bad habits inherited from others / I don't know. Just now learning from your helpful answer here on the difference. It appears \mathrm does not show the problem that \textrm does, so there is the answer to part 2 of my question. — gerrit, Jul 04 '16 at 14:09
I have the same versions of TeX Live, LuaTeX, and beamer, but an older version of breqn (v0.98b), which does NOT have this problem. — JPi, Jul 04 '16 at 14:23
Do I need to describe the version of any additional software? — gerrit, Jul 04 '16 at 14:30
@HeikoOberdiek The luatex tag was not yet there when I tried :) — samcarter_is_at_topanswers.xyz, Jul 04 '16 at 15:14

score 4 · Accepted Answer · answered Jul 04 '16 at 14:29

The problem is triggered, if the redefinition of \mathchoice in package mathstyle meets LuaTeX's \mathstyle, from package mathstyle:

\def\mathchoice{%
  \relax\ifcase\mathstyle
    \expandafter\@firstoffour % Display
  \or
    \expandafter\@firstoffour % Cramped display
  \or
    \expandafter\@secondoffour % Text
  \or
    \expandafter\@secondoffour % Cramped text
  \or
    \expandafter\@thirdoffour % Script
  \or
    \expandafter\@thirdoffour % Cramped script
  \else
    \expandafter\@fourthoffour % (Cramped) Scriptscript
  \fi
}

\ifcase expects a number and continues the expansion until a token is found, which does not contain to a number (non-digit). Thus the first \expandafter of case "0" for "display style" is called at the wrong time, when the number is still being read. A \relax stops that and should fix the issue:

\documentclass{beamer}
\usepackage{breqn}

\makeatletter
\def\mathchoice{%
  \relax\ifcase\mathstyle\relax
    \expandafter\@firstoffour % Display
  \or
    \expandafter\@firstoffour % Cramped display
  \or
    \expandafter\@secondoffour % Text
  \or
    \expandafter\@secondoffour % Cramped text
  \or
    \expandafter\@thirdoffour % Script
  \or
    \expandafter\@thirdoffour % Cramped script
  \else
    \expandafter\@fourthoffour % (Cramped) Scriptscript
  \fi
}
\makeatother

\newcommand{\spam}{\textrm{Spam}}
\begin{document}
\begin{frame}
\centering

$\displaystyle \mathchoice{D}{T}{S}{s}$

\[ \spam \]
\end{frame}
\end{document}

Lualatex beamer breqn: Text appears four timestimestimestimes, why and how to mitigate?

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