beamer's \only causes shifts

Question

When using \only with LaTeXs beamer package, distances seem to change:

\documentclass{beamer}
\begin{document}
\begin{frame}[t]{}
\begin{columns}[T]
    \fbox{%
    \begin{column}{0.6\textwidth}
        Test
        \only<-4>{[a]}
        \only<5>{[b]}
        \only<6>{[c]}
        \only<7>{[d]}
        \only<4->{\fbox{\textbf{Test}}}
        \only<5->{%
            \begin{itemize}
                \item b
                \only<6->{\item c}
                \only<7->{\item d}
            \end{itemize}}
    \end{column}}
    \fbox{%
    \begin{column}{0.4\textwidth}
        \includegraphics[width=\textwidth]{example-image-a}
    \end{column}}
\end{columns}
\end{frame}
\end{document}

The second Test slides along from frame to frame (all slides overlayed):

What is happening here?

(P.S. And why are the columns not centered?)

Answer 1

• the Test box moves because each line ending acts like a space. To avoid this, you can protect the line endings with a % sign

• Beamer does not like other content in the columns environment other than columns. You can make the columns centred by moving the \fbox inside the columns:

---

\documentclass{beamer}
\begin{document}
\begin{frame}[t]{}
\begin{columns}[T]
    \begin{column}{0.6\textwidth}
    \fbox{\begin{minipage}{\dimexpr\textwidth-2\fboxsep-2\fboxrule}
            Test
        \only<-4>{[a]}%
        \only<5>{[b]}%
        \only<6>{[c]}%
        \only<7>{[d]}%
        \only<4->{\fbox{\textbf{Test}}}%
        \only<5->{%
            \begin{itemize}
                \item b
                \only<6->{\item c}
                \only<7->{\item d}
            \end{itemize}}%
                \end{minipage}}%
    \end{column}
    \begin{column}{0.4\textwidth}
            \fbox{\includegraphics[width=\dimexpr\textwidth-2\fboxsep-2\fboxrule]{example-image-a}}%
    \end{column}
\end{columns}
\end{frame}
\end{document}