slide dependent subitemize in beamer

Question

I'm not sure if I do it well but I have what follows:

\documentclass[10pt,fleqn,t]{beamer}
\begin{document}
\frame[t]{
\begin{block}{Title}
     \begin{itemize}
     \item text 1
     \only<1>{%
         \begin{itemize}%
            \item subtext 1
            \item subtext 2
         \end{itemize}}
 \only<2-3>{\item text 2}
 \only<3>{\item text 3}
     \end{itemize}
\end{block}}
\end{document}

When the items move vertically when moving forward in the presentation: is there a way to fix this? (I've tried \onslide instead but it freezes vertical space which I do not want).

Added: (by Andrew Stacey) To clarify the problem: it is that in the above document, the text 1 (which is shown on all slides) shifts vertically ever-so-slightly-but-noticeably between frames 1 and 2. As it is the first item in the list and there is plenty of space on the slide, this doesn't not seem the right behaviour.

Answer 1

I conjecture that this is to do with when TeX knows that the paragraph containing the text 1 has been finished. The inner itemize starts a new paragraph and so shouldn't affect the spacing about the previous \item, but it does. Exactly what shape or form this confusion takes, I don't know. One could well envision this as a bug.

However, despite not knowing exactly what is going on, I can suggest a simple fix: ensure that the paragraphs are clearly separated. Viz, put a blank line after the jumping \item.

\documentclass[10pt,fleqn,t]{beamer}
%\url{http://tex.stackexchange.com/q/22809/86}
\begin{document}
\frame[t]{
\begin{block}{Title}
     \begin{itemize}
     \item text 1

     \only<1>{%
         \begin{itemize}%
            \item subtext 1
            \item subtext 2
         \end{itemize}}
 \only<2-3>{\item text 2}
 \only<3>{\item text 3}
     \end{itemize}
\end{block}}
\end{document}

For consistency, but for nothing else, you could have a blank line after the sub-itemized list.

Interestingly, I only got this behaviour with itemize and enumerate and not with any other environments that I tried (including description). Moreover, it is nothing to do with the \only since the following code shows the issue:

\documentclass[10pt,fleqn,t]{beamer}
%\url{http://tex.stackexchange.com/q/22809/86}
\begin{document}
\frame[t]{
\begin{block}{Title}
     \begin{itemize}
     \item text 1
         \begin{itemize}%
            \item subtext 1
            \item subtext 2
         \end{itemize}
     \end{itemize}
\end{block}}

\frame[t]{
\begin{block}{Title}
     \begin{itemize}
     \item text 1
  \item text 2
     \end{itemize}
\end{block}}
\end{document}

and the same blank line fixes it.

Answer 2

If you don't like the output using \item<2-3> text 2 then you can try use an empty "space kipper", that is:

\only<1,4->{\item \mbox{}}
\only<2-3>{\item text 2}