Unexpected output using tikz/textpos

Question

The following code (MWE)

\documentclass[a4paper]{article}

\usepackage{tikz}
\usepackage[absolute]{textpos}

\usetikzlibrary{external}
\tikzexternalize

\begin{document}

\begin{textblock}{4}[0,0](1.3,2.5)
\begin{center}
SOME TITLE TEXT SOME TITLE TEXT\\
SOME TITLE TEXT SOME TITLE TEXT
\end{center}
\end{textblock}
\thispagestyle{empty}
\mbox{} \clearpage  %flush this page

\begin{figure}
\begin{tikzpicture}
  \path[draw] (20mm,150mm) -- (20mm,250mm);
  \path[draw] (100mm,150mm) -- (100mm,250mm);
\end{tikzpicture}

\end{figure}

\end{document}

is expected to produce two pages;

the first one containing eg some title text and the second one two parallel lines.

Instead in the second page the first one is appended (take a look at the picture)

I am using pdflatex from MikTeX 2.9 (updated) and compiling with -enable-write18.

What's wrong?

Answer 1

The problem is that both textpos package and externalize library play games with TeX shipout mechanism.

Supposedly, externalize should ignore all text outside of the tikzpicture environments, which includes the text in the textblock environment. However, textpos in absolute mode includes this text inside a box which is shipped at the end of the page, apparently without tikz knowing it, and thus interfering in the "page" that externalize is trying to build.

The hack I've found is to empty this textpos box after using it. In this particular example, the following MWE works, but I don't know if it is general enough. Try it in your actual use case and tell us.

\documentclass[a4paper]{article}

\usepackage{tikz}
\usepackage[absolute]{textpos}

\makeatletter
\def\cleartextposbox{\global\setbox\TP@holdbox\vbox{}}
\makeatother

\usetikzlibrary{external}

\tikzexternalize
\begin{document}
\begin{textblock}{4}[0,0](1.3,2.5)
\begin{center}
SOME TITLE TEXT SOME TITLE TEXT\\
SOME TITLE TEXT SOME TITLE TEXT
\end{center}
\end{textblock}
\thispagestyle{empty}
\null\newpage\cleartextposbox   % <----- 

\begin{figure}
\begin{tikzpicture}
 \path[draw] (20mm,150mm) -- (20mm,250mm);
 \path[draw] (100mm,150mm) -- (100mm,250mm);
\end{tikzpicture}
\end{figure}

\end{document}