Shadow of boxes not long enough on a3paper

Hello!

I use beamer to make a conference poster. The plan is to do it on A3 
paper and scale it to A1 (or something) when printing so that I don't 
have to change any font sizes.

There is only one problem so far: when using "beamerthemeshadow", the 
shadow of \begin{block} ... \end{block}-boxes is not long enough for the 
width of A3 landscape paper, see the following example:

\documentclass{beamer}

\geometry{a3paper,landscape}
\usepackage{beamerthemeshadow}

\begin{document}
\begin{frame} \frametitle{Foo}
   \begin{block}{Bar}
     Here is the problem: the grey shadow does not extend over the
     whole length of the box.
   \end{block}
\end{frame}
\end{document}

How can I extend the grey shadow?

Thanks and best regards,

Svjetlana

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