I'll get straight to the point. MWE:
\documentclass{beamer}
\usetheme{metropolis}
\begin{document}
\begin{frame}[t]{title}
\begin{itemize}
\item 0
\only<+>{
\item a
}
\only<+>{
\item b
\item c
}
\only<+>{
\item d
\item e
\item f
\item
ggggggggggggggggggggggggggggggggggggggggg
ggggggggggggggggggggggggggggggggggggggggg
ggggggggggggggggggggggggggggggggggggggggg
ggggggggggggggggggggggggggggggggggggggggg
ggggggggggggggggggggggggggggggggggggggggg
ggggggggggggggggggggggggggggggggggggggggg
ggggggggggggggggggggggggggggggggggggggggg
ggggggggggggggggggggggggggggggggggggggggg
}
\end{itemize}
\end{frame}
\end{document}
I.e. 0 stays at the top all the time, a is on the first slide then leaves, b and c come and leave on slide two and so forth.
The problem is, that d-g are very long and thus on the third slide item 0 floats very slightly to the top (less than say 0.5em). However, I still have enough space at the bottom of the slide to not have this happen. Frame option [t] does not seem to help. This is super irritating as the third slide seems to be something totally new to the eye at first.
I'm sure there is a very easy fix to this. I appreciate any help. Many thanks!