The problem I have is simple to describe: the height of a beamer slide title does not seem to be subtracted from \textheight, leaving me with no idea how much space is left for the actual content.
A quick example:
\documentclass{beamer}
\usepackage{tikz}
\begin{document}
\begin{frame}{Title}
\begin{tikzpicture}
\draw[red] (0,0) rectangle (\textwidth,\textheight);
\draw[blue] (0,\textheight) -- (\textwidth,0)
(0,0) -- (\textwidth, \textheight);
\end{tikzpicture}
\end{frame}
\end{document}
So, is this a bug, or am I using it wrong or misunderstanding it? How would I go on about with drawing the above rectangle with the correct height?

\textheightis a lot taller inside tikzpicture than it is on the beamer frame. No idea why – daleif Aug 03 '17 at 12:44\begin{tikzpicture}[overlay, remember picture] \draw[red] (current page.south west) rectangle (\textwidth,0.3\textheight);\end{tikzpicture}. Why do I need such a small height for the rectangle for the square to be seen. – daleif Aug 03 '17 at 12:47