\insertframetitle can't be used inside \MakeUppercase if there is a framebreak.

[u-fischer]

The definition of \insertframetitle tries to call a template if a framebreak is detected, and this breaks in \MakeUppercase.

! Argument of \XC@col@rlet has an extra }.
<inserted text> 
                \par 
l.158 \end{frame}

Perhaps the definition could be made more safe by protecting the template with a \CaseSwitch?

\documentclass{beamer}
\makeatletter
\long\def\beamer@@frametitle[#1]#2{%
  \ifblank{#2}{}{%
    \gdef\insertframetitle{{#2\ifnum\beamer@autobreakcount>0\relax{}\space
     \CaseSwitch
       {\usebeamertemplate*{frametitle continuation}}
       {\usebeamertemplate*{frametitle continuation}}
       {\usebeamertemplate*{frametitle continuation}}
       {\usebeamertemplate*{frametitle continuation}}%
      \fi}}%
  \gdef\beamer@frametitle{#2}%
  \gdef\beamer@shortframetitle{#1}%
}%
}
\makeatother

\defbeamertemplate*{frametitle}{uppercase}{\MakeUppercase{\insertframetitle}}
\begin{document}

\begin{frame}[allowframebreaks]
\frametitle{Flussdiagramm}

aaaa

\framebreak

bbb

\end{frame}
\end{document}

[samcarter]

@u-fischer Thanks for reporting!

[samcarter]

Reopening this issue for now as this would break the metropolis theme.

The proposed fix is stored in https://github.com/josephwright/beamer/tree/802-Uppercase-frametitle and a PR is opened for metropolis at matze/mtheme#391

[samcarter]

The Trigon theme suffers from the same problem. I left a heads-up issue at https://gitlab.com/thlamb/beamertheme-trigon/-/issues/7

Update: The author is informed and very helpfully agreed to prepare an update for when this goes live for beamer. Half of the problem solved :)

[samcarter]

@josephwright There seems to be no reaction from metropolis, so I was looking into your suggestion of doing some first aid via filehooks. My current approach is to revert back to the old definition before metropolis, let it patch whatever it wants and then overwrite the definition again at the end of the file. Is this what you had in mind or is there some better solution?

\documentclass{beamer}

\makeatletter
\AddToHook{file/beamerfontthememetropolis.sty/before}{%
  \renewcommand<>\frametitle{\alt#1{\@dblarg\beamer@@frametitle}{\beamer@gobbleoptional}}
  \long\def\beamer@@frametitle[#1]#2{%
    \ifblank{#2}{}{%
      \gdef\insertframetitle{{#2\ifnum\beamer@autobreakcount>0\relax{}\space\usebeamertemplate*{frametitle continuation}\fi}}%
    \gdef\beamer@frametitle{#2}%
    \gdef\beamer@shortframetitle{#1}%
  }%
  }
}

\AddToHook{file/beamerfontthememetropolis.sty/after}{%
  \renewcommand<>\frametitle{\alt#1{\@dblarg\beamer@@frametitle}{\beamer@gobbleoptional}}
  \long\def\beamer@@frametitle[#1]#2{%
    \ifblank{#2}{}{%
      \gdef\insertframetitle{{%
        \metropolis@frametitleformat{#2}%
        \ifnum\beamer@autobreakcount>0
          \relax{}\space%
          \beamer@insertframetitlecontinuation%
        \fi%
      }}%
    \gdef\beamer@frametitle{#2}%
    \gdef\beamer@shortframetitle{#1}%
    }%
  }
}
\makeatother

\usetheme{metropolis}

\begin{document}

\begin{frame}[allowframebreaks]
\frametitle{Flussdiagramm}

aaaa

\framebreak

bbb

\end{frame}
\end{document}
	
\end{document}

[josephwright]

@samcarter Looks good to me

[samcarter]

@josephwright Thanks! I'll merge the changes

[samcarter]

@josephwright Could you coordinate with @lambtho12 about the timing of the next release so that the trigon theme can be updated accordingly?

[lambtho12]

I will prepare the patch for Trigon sometime next week in a separate branch.
I subscribed to the RSS feed for beamer on CTAN so I will be notified directly when the update is available. I will then merge the patch and publish Trigon as well.

[lambtho12]

FYI, Trigon patch is ready. I will merge as soon as new version of beamer hits CTAN.

[samcarter]

@lambtho12 FYI: Joseph released beamer v3.69 2023-02-20 earlier today. It should currently make its way around the ctan mirrors and be available there within the next 24 hours or so.

[lambtho12]

Awesome. Thanks for the notif. I will package an publish Trigon this evening after work.

[lambtho12]

Just published fix on CTAN as well. Works as expected with latest Beamer.