\centering doesn't work in plainframes

[Willem3141]

This doesn't center the text:

\begin{plainframe}{}
    \centering
    bla bla bla\ldots
\end{plainframe}

On the other hand, this does:

\begin{frame}{}
    \centering
    bla bla bla\ldots
\end{frame}

I guess we should find out why this happens, or at least document it... 😉

(Note that \begin{center} ... \end{center} works in both environments, so it is not a huge issue 😄)