Avoiding jumps when using beamer overlays

Question

I have the following piece of code in a beamer slide to put the curly brackets under the equation on the second slide:

\begin{equation*}
    a = \alt<1>{b+c+d}{\underbrace{b+c+d}_{\text{some text}}}.
\end{equation*}

This causes jumps in the slides. I was wondering how I should avoid this?

I tried to wrap the equation inside an overprint environment (as suggested in Avoiding jumping frames in beamer) but it didn't help.

Thanks for your helps in advance!

Answer 1

This creates a new environment overlayauto which works like overlayarea but computes the height, width and depth from the data.

Note: this is like putting an \mbox in a math environment in that you need to use $...$ inside.

\documentclass{beamer}
\usepackage{environ}

% used to replace \only
\def\testonly<#1>#2{\setbox0=\hbox{#2}%
  \ifdim\ht0>\maxheight\relax\maxheight=\ht0\fi
  \ifdim\dp0>\maxdepth\relax\maxdepth=\dp0\fi
  \ifdim\wd0>\maxwidth\relax\maxwidth=\wd0\fi
}
\NewEnviron{overlayauto}[1][\normalfont\raggedright]% #1 = minipage optons
{% get dimentsions from data
  \dimendef\maxheight=0
  \dimendef\maxwidth=1
  \dimendef\maxdepth=2
  \maxheight=0pt
  \maxdepth=0pt
  \maxwidth=0pt
  \let\temp=\only
  \let\only=\testonly
  \BODY
  \let\only=\temp
  \raisebox{0pt}[\maxheight][\maxdepth]%
    {\begin{minipage}[t]{\maxwidth}
      #1\BODY
    \end{minipage}}
}
\begin{document}
\begin{frame}
above
\begin{equation*}
a=\begin{overlayauto}
\only<1>{$b+c+d$}%
\only<2>{$\underbrace{b+c+d}_{\text{some text}}$}
\only<3>{$\overbrace{b+c+d}^{\text{some text}}$}
\end{overlayauto}
\end{equation*}
below
\end{frame}
\end{document}