Beamer overlay only/visible

Question

I am trying to change a formula between two slides. Here is an MWE

\documentclass{beamer}

\begin{document}

\begin{frame}{Title}
Something before\\
\bigskip
$%
B\left(%
\only<1>{A_2[i]}%
\only<2>{A_L[i]}%
\right) =%
\only<1>{C_1 + C_2}%
\visible<2>{\displaystyle\sum_{k=1}^{L} C_k}%
\quad \text{text here }%
\only<1>{a,b,c,}%
\only<2>{a,b,c,\ldots,z}
$

\end{frame}

\end{document}

As you can see, there is a problem with the position of "text here" in the first slide. If I put only instead of visible, that problem is solved but then everything moves during the transition. Is there a way to solve both issues?

Answer 1

The command \vphantom in useful in this case. It will reserve the vertical space required by the sum and thus prevent "wobbling" of the formula during overlays.

\documentclass{beamer}

\begin{document}

\begin{frame}{Title}
Something before\\
\bigskip
$%
B\left(%
\only<1>{A_2[i]}%
\only<2>{A_L[i]}%
\right) =%
\only<1>{C_1 + C_2 \vphantom{\displaystyle\sum_{k=1}^{L} C_k}}%
\only<2>{\displaystyle\sum_{k=1}^{L} C_k}%
\quad \text{text here }%
\only<1>{a,b,c,}%
\only<2>{a,b,c,\ldots,z}
$

\end{frame}

\end{document}