Beamer interaction remove footline [duplicate]

Question

Can anyone explain why the MWE below is bad? When I compile it, there is no footline on the first page!?!

I use the <+> and <+-> in order to not having to explicitly having to specify the various overlays.

\documentclass{beamer}
\usepackage[T1]{fontenc}
\usepackage{fix-cm}
\usetheme{Copenhagen}
\author{Author}
\institute{Department}
\title{Title}
\begin{document}
\begin{frame}
  \frametitle{Frame title}

  \begin{onlyenv}<+>
    Overlay 1, just one
  \end{onlyenv}

  \begin{onlyenv}<+->
    Overlay 2

    \onslide<+->

    more text
  \end{onlyenv}
\end{frame}
\end{document}

Answer 1

Understanding this requires a subtle reading of the beamer manual. (Note: this is Till's text, not mine!) For beamer 3.36, on page 80 one finds the description of \onslide. The given syntax is

\onslide⟨modifier⟩<⟨overlay specification⟩>{⟨text⟩}

where the {⟨text⟩} argument is optional. There is then detail of what happens if there is no {⟨text⟩}, and importantly

You need not call this command in the same TeX group, its effect transcends block groups.

That's important because you've said \onslide<+->, which in the example means <2->. What then happens is that this applies to everything on the frame after this point, including the picture environment used to draw the footer! Not all footer-like elements follow (internally) at the end of the frame, so things like the navigation symbols survive.

If you use the optional {⟨text⟩} then \onslide behaves like \uncover, which does obey groupings.