\hrule within \textblock (from textpos package) does not appear in the output

Question

My code:

\documentclass{beamer}
\usepackage[absolute,overlay]{textpos}
\begin{document}

\begin{frame}[fragile,t]
\frametitle{Demo}

\begin{textblock*}{\textwidth}(20mm,20mm)
\vspace{5mm}\hrule height 1mm\vspace{5mm}
\end{textblock*}

\end{frame}

\end{document}

Output:

Why does the \hrule not appear in the output? How can I fix it?

Answer 1

The textblock* environment uses the properties of the content to determine the size of the box (even though the textpos manual states that the width of textpos* is absolute). The TeX primitive \hrule takes as width the width of the of the vertical box that encloses the \hrule instruction (see Why does a \hrule need text around it to be drawn?), which is empty in this case.

Possible solutions are to use \leavevmode (that sets the width of the box to \textwidth, see the linked question), or specify the width of the \hrule manually, or to use the command \rule with a specified width.

MWE below, with some examples of how textpos* handles different types of content for illustration purposes (using the showboxes package option).

\documentclass{beamer}
\usepackage[absolute,overlay,showboxes]{textpos}
\begin{document}

\begin{frame}[fragile,t]
\frametitle{Demo}
\color{green}\vspace{5mm}\hrule height 1mm\vspace{5mm}
\begin{textblock*}{\textwidth}(10mm,20mm)
\vspace{2mm}\hrule height 1mm\vspace{2mm}% zero width
\end{textblock*}
\begin{textblock*}{\textwidth}(10mm,30mm)
\vspace{2mm}\leavevmode\hrule height 1mm\vspace{2mm}
\end{textblock*}
\begin{textblock*}{\textwidth}(10mm,40mm)
\color{red}\vspace{2mm}\hrule height 1mm width \textwidth\vspace{2mm}
\end{textblock*}
\begin{textblock*}{\textwidth}(10mm,50mm)
\color{blue}\rule{\textwidth}{1mm}
\end{textblock*}
%further examples of textblock*
\begin{textblock*}{\textwidth}(10mm,60mm)% width of the character
\phantom{X}
\end{textblock*}
\begin{textblock*}{\textwidth}(10mm,70mm)% does not show

\end{textblock*}
\begin{textblock*}{\textwidth}(10mm,80mm)% full width
\color{black}X
\end{textblock*}
\end{frame}

\end{document}