Upright maths in display mode, but italics in align environment in beamer class

Question

I use mathastext like this

\renewcommand{\familydefault}{\sfdefault}
\usepackage[defaultmathsizes]{mathastext}
\renewcommand{\familydefault}{\rmdefault}

This gives me upright math throughout my document in an article class document, but in a beamer class document I end up with upright math in display math mode \[ ... \], but in the align environment \begin{align} ... \end{align} the math is italic.

How do I get upright math in an align environment as well?

---

\documentclass[professionalfonts]{beamer}
\usepackage[T1]{fontenc}
\usepackage{libertine}
\renewcommand{\familydefault}{\sfdefault}
\usepackage[defaultmathsizes]{mathastext}
\renewcommand{\familydefault}{\rmdefault}

\begin{document}
\begin{frame}

Math in display
\[
abc
\]

Text...

Math in align
\begin{align}
abc
\end{align}

\end{frame}
\end{document}

Answer 1

This is a rewrite of my first answer. There are two options:

1. Use the professionalfont beamer class option and mathastext exactly as in your code, but notice the spelling without a final s. Alternatively, and as is recommended by a warning in the log file, issue rather in the preamble \usefonttheme{professionalfonts} (with a final s!).

2. Do not use the professionalfont class option nor \usefonttheme{professionalfonts}, do not load package mathastext, and exploit the fact that then the beamer class by itself already reassigns letters and digits to the document sans text font. There is perhaps an option somewhere in beamer to tell it to get the letters upright. But, not knowing it I provide a hack which has the desired effect.

The difference between the two options is in the treatment of characters such as =, ? and ;. In option 1, they will be from Biolinum (this is done by mathastext package), in option 2 they will be from the default math (usually Computer Modern) fonts.

Option 1 (professionalfont+mathastext)

Code for option 1:

%\documentclass[professionalfont]{beamer}
%  or better (according to a warning in log file if the above):
\documentclass {beamer}
\usefonttheme {professionalfonts}

\usepackage[T1]{fontenc}
\usepackage{libertine}

\renewcommand{\familydefault}{\sfdefault}
\usepackage[defaultmathsizes]{mathastext}
\renewcommand{\familydefault}{\rmdefault}
\begin{document}
\begin{frame}

  This is serif text, $sans math:E=mc^2$ (we see it is upright and sans), and
  then display math which is also upright and sans:
  \[
  E=mc^2=abc;XYZ=tuv=TUV<123?
  \]

  Some serif text and then display math (with align)
  \begin{align} 
   E=mc^2=abc;XYZ=tuv=TUV<123?
  \end{align}

\end{frame}
\end{document}

Option 2: hack, but no mathastext

Code for option 2:

\documentclass{beamer}
\usepackage[T1]{fontenc}
\usepackage{libertine}
\usefonttheme[stillsansserifmath]{serif}

\makeatletter
\AtBeginDocument{\begingroup
   \count0 \sympureletters
   \count2 \symnumbers
  \@tfor\x:=abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ\do
  {\global\expandafter\mathcode\expandafter`\x=\numexpr\expandafter\mathcode\expandafter`\x-\count0*256+\count2*256\relax}\endgroup}
\makeatother

\begin{document}
\begin{frame}

  This is serif text, $sans math:E=mc^2$ (we see it is upright and sans), and
  then display math which is also upright and sans:
  \[
  E=mc^2=abc;XYZ=tuv=TUV<123?
  \]

  Some serif text and then display math (with align)
  \begin{align} 
   E=mc^2=abc;XYZ=tuv=TUV<123?
  \end{align}

\end{frame}
\end{document}

---

As an aside, not directly related to the OP question, I have noticed differences between pdflatex, and lualatex/xelatex with the following code.

\documentclass{beamer}
\usepackage{libertine}
\begin{document}
\begin{frame}
  \showboxdepth\maxdimen
  \showboxbreadth\maxdimen
  \setbox0\hbox{$=$}\showbox0\box0
\end{frame}
\end{document}

The font is cmss with pdflatex but cmr with lualatex/xetex.

pdftex:
> \box0=
\hbox(4.05148+0.0)x8.5167
.\mathon
.\OT1/cmss/m/n/10.95 =
.\mathoff

lualatex:
> \box0=
\hbox(4.01727+0.0)x8.5167, direction TLT
.\mathon
.\OT1/cmr/m/n/10.95 =
.\mathoff

xetex:
> \box0=
\hbox(4.01727+0.0)x8.5167
.\mathon
.\OT1/cmr/m/n/10.95 =
.\mathoff