action aware \messagebox

beamerthemelines.sty

@@ -486,19 +486,19 @@
     {\usebeamercolor[fg]{message symbol}\usebeamertemplate{message symbol}}%
     \strut\insertmessage
   \end{beamercolorbox}
-  \nointerlineskip%
   \ifbeamercolorempty[fg]{message line bottom}{}{%
+    \nointerlineskip%
     \begin{beamercolorbox}[wd=\columnwidth,hd=#2]{message line bottom}
       \rule{\columnwidth}{#2}
-    \end{beamercolorbox}
-    \nointerlineskip}%
+    \end{beamercolorbox}}%
+  \medskip
 }
 
 \setbeamertemplate{message}[lines]{0.5pt}{0.5pt}
 
-\newcommand{\messagebox}[1]{%
-  \def\insertmessage{#1}%
-  \usebeamertemplate*{message}
+\newcommand<>{\messagebox}[1]{%
+  \only#2{\def\insertmessage{#1}%
+    \usebeamertemplate*{message}}
 }