There's no automatic line break after "Theorem.", "Lemmas.", etc.,
these days, but there is still one (with the wrong spacing!) after
"Proof." Here we override the beamer template to make "Proof." work
like the others.
% Spacing is nice.
\setlength{\parskip}{1em}
% Spacing is nice.
\setlength{\parskip}{1em}
+
+% Don't break the line that "Proof." is on.
+\setbeamertemplate{proof begin}{
+ \usebeamerfont*{block title}
+ \insertproofname
+ \usebeamerfont*{block body}
+}
+\setbeamertemplate{proof end}{}