fixed unstable text position in sidebar theme by compensating for the…

… space automatically inserted after the frametitle in https://github.com/josephwright/beamer/blob/main/base/beamerbaseframe.sty#L126

Test file:
```
\documentclass[10pt,c]{beamer}

\usetheme{Berkeley}

\begin{document}

\begin{frame}
Some content.
\end{frame}
\begin{frame}
\frametitle{Title}
Some content.
\end{frame}
\begin{frame}
Some content.
\end{frame}

\end{document}
```

CHANGELOG.md

@@ -27,6 +27,7 @@ a major and minor version only.
 - fixed missing `\newline` if ragged2e is used together with beamerarticle (#900)
 - defined `\fnum@table` to avoid breaking longtable (#905)
 - fixed: other sections would show the template for "miniframe in other subsection" instead of "mini frame in other section"
+- fixed unstable text position in sidebar theme by compensating for the space automatically inserted after the frametitle in https://github.com/josephwright/beamer/blob/main/base/beamerbaseframe.sty#L126
 
 ## [v3.71]
 

base/themes/outer/beamerouterthemesidebar.sty

@@ -64,6 +64,7 @@
       \vskip-1em%
       \vfil
     }%
+    \vskip-0.2em%
   }
 
   \defbeamertemplate*{headline}{sidebar theme}