How to position a TikZ node in an arbitrary position inside a beamer slide's main area

Question

The following question is related to this one.

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I'd like to be able to position a TikZ node in an arbitrary position inside the "main area" of a beamer slide, i.e. inside the white rectangle in the example below.

\documentclass{beamer}
\usetheme{Warsaw}
\begin{document}
\section{A section}
\begin{frame}{Title}
\end{frame}
\end{document}

In order to do so, I need to have access to one of several collections of information:

1. The heights of the various articles that go in the top and bottom of the slide:

   • the section banner
   • the title banner
   • the navigation ruler
   • the banner along to bottom edge of the slide

2. The distances of the four corners of the main area from the edge of the page closest to them, respectively.

3. The distance of one of the four corners of the main area from the edge of the page closest to it, together with the height of the main area.

4. An object similar to TikZ's current page that represents the main area, so it would be possible to write something like \draw[fill=red] (main area.north west) circle (1pt); to get a filled red circle at the top left corner of the main area.

In either case, I'd also like to know the height of the shade under the title banner.

Note that the height of the various articles mentioned in the first point above is dynamic, and each of them can be hidden separately from the others.

1. How can I gain access to a sufficient amount of information that would enable me to position a TikZ picture at an arbitrary position in a slide's main area?
2. Is there a general answer to the previous question irrespective of theme? This would be preferrable over an answer that relies on the Warsaw theme.

Answer 1

The height of the text area is unknown, or at least is not equal to \textheight. The text area either overlaps the bottom of the slide, or it doesn't. there is no way to know a priori. I don't think beamer even calculates the available space. It just applies \vfill above and below and hopes.

Using \tikzmark you can locate of the NW and SE corners, or even compute offsets relative to (current page) anchors.

\documentclass{beamer}
\usetheme{Warsaw}
\usepackage{tikz}
\usetikzlibrary{tikzmark,calc}
\begin{document}
\section{A section}
\begin{frame}{Title}
\rule{\textwidth}{16\baselineskip}
\end{frame}
\begin{frame}{Title}
\parbox[c][16\baselineskip][s]{\textwidth}{%
\tikzmark{topleft}\par
\vfill
\hfill\tikzmark{bottomright}}%
\tikz[remember picture, overlay]{\fill[black]
   (pic cs:topleft) rectangle (pic cs:bottomright);}
\end{frame}
\end{document}