Beamer uncover decoration bug

Question

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{pgf,tikz}
\usepackage{tikz}
\begin{document}

\frame{
\usetikzlibrary{decorations.pathmorphing}
\tikzstyle{kringel}=[decoration={random steps}]
\begin{tikzpicture}
 \draw[black] (3,2.5) decorate [kringel] {-- (2,5)};
\end{tikzpicture}

%\uncover<2->{test}
}
\end{document}

Compiles without any problems. If I replace

%\uncover<2->{test}

with

\uncover<2->{test}

then it does not compile and gives the following error:

./test.tex:18: Package tikz Error: You need to load a decoration library.

If I move the two lines

\usetikzlibrary{decorations.pathmorphing}
\tikzstyle{kringel}=[decoration={random steps}]

into the document header, i.e., before

\begin{document}

then it compiles and works. Can someone explain to me what is happening here?

Answer 1

What it boils down to in the very end is that you probably shouldn't try to use \usetikzlibrary in a group.

With a bit of paring down, the error already occurs in

\documentclass{article}
\usepackage{pgf,tikz}
\begin{document}

\begingroup
\usetikzlibrary{decorations.pathmorphing}
\endgroup

\tikzstyle{kringel}=[decoration={random steps}]
\begin{tikzpicture}
 \draw[black] (3,2.5) decorate [kringel] {-- (2,5)};
\end{tikzpicture}

\end{document}

As soon as you have more than one overlay in a frame, as by using a <2->, the whole frame is parsed several times, throwing away the stuff that is not applicable to the current overlay. Every iteration is done in a group, to limit side effects seeping out. It now appears (I've not gone down that rabbithole) that \usetikzlibrary is clever enough to not load anything twice. So your library is loaded for overlay <1>, not loaded for overlay <2>, but its definitions aren't there because they weren't global.