Why is TikZ overlay not working in this baposter?

Question

Consider following baposter where I added an overlayed TikZ figure to draw an arrow on top of the poster. (I observed that adding the given TikZ figure inside a box adds excess vertical spacing below its content, so I tried to put the TikZ figure after the last box.)

\documentclass[a0paper,portrait]{baposter}
\usepackage{tikz}
\definecolor{bordercol}{RGB}{40,40,40}
\definecolor{headercol1}{RGB}{186,215,230}
\definecolor{headercol2}{RGB}{80,80,80}
\definecolor{headerfontcol}{RGB}{0,0,0}
\definecolor{boxcolor}{RGB}{186,215,230}
\begin{document}
\background{
}
\begin{poster}{
    grid=false, 
    borderColor=bordercol,
    headerColorOne=headercol1,
    headerColorTwo=headercol2,
    headerFontColor=headerfontcol,
    boxColorOne=boxcolor,
    headershape=roundedright,
    headerfont=\Large\sf\bf,
    textborder=rectangle,
    background=user,
    headerborder=open,
    boxshade=plain,
    headerheight=3cm
}
%%% Eye Cacther %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
{
}
%%% Title %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
{\sf\bf
    \includegraphics[height=3cm]{pix/elte_logo}\
    Can I lower this title?
}
%%% Authors %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
{
    \vspace{1em} Pr. Oblematic\
    {Affiliation \ Long affiliation \ Very long affiliation}
}
%%% Logo %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
{
}
\headerbox{Problem}{name=problem1,column=0,row=0}{
}
\headerbox{Solution}{name=solution1,column=0,below=problem1,above=bottom}{
}
\headerbox{Problem}{name=problem2,span=2,column=1,row=0}{
}
\headerbox{Solution}{name=solution2,span=2,column=1,below=problem2,above=bottom}{
}
\begin{tikzpicture}[remember picture, overlay]
\draw[red,->] (current page.center) -- (current page.center)++(1cm, 1cm);
\end{tikzpicture}
\end{poster}
\end{document}

(Adapted from the ECCS2011 tutorial that comes with baposter.)

However, I get following result. What happened? How can I correctly overlay a TikZ picture over a given poster?

Answer 1

1. The path

   (current page.center) -- (current page.center)++(1cm, 1cm)
   

   describes a straight line of length zero from and to the page's center and then a move to operation relative to the center (that's one +) and also moves the new origin for subsequent relative coordinates to that point (that's the other +).

2. As Sam notes baposter uses a TikZ picture internally, so just putting another TikZ picture inside it breaks everything.

3. You can't just put the new TikZ picture after the poster environment because … pagebreak.

4. You can however, put the new TikZ picture in the foreground of the current page via the hook shipout/foreground by adding before the poster environment.

   \AddToHookNext{shipout/foreground}{%
     \begin{tikzpicture}[remember picture, overlay]
     \draw[red,->] (current page.center) -- ++(1cm, 1cm);
     \end{tikzpicture}%
   }
   

5. You can also use the TikZ picture used by poster by adding

   \tikzset{
     remember picture,
     execute at end picture={
   %    \tikzset{overlay}% maybe
       \draw[red, ->] (current page.center) -- ++(1cm, 1cm);
     }
   }
   

   in the poster environment.

Code

\documentclass[a0paper,portrait]{baposter}
\definecolor{bordercol}{RGB}{40,40,40}
\definecolor{headercol1}{RGB}{186,215,230}
\definecolor{headercol2}{RGB}{80,80,80}
\definecolor{headerfontcol}{RGB}{0,0,0}
\definecolor{boxcolor}{RGB}{186,215,230}
\begin{document}
\AddToHookNext{shipout/foreground}{%
  \begin{tikzpicture}[remember picture, overlay]
  \draw[blue, ->] (current page.center) -- ++(-1cm, -1cm);
  \end{tikzpicture}%
}
\background{}
\begin{poster}{
    grid=false,
    borderColor=bordercol,
    headerColorOne=headercol1,
    headerColorTwo=headercol2,
    headerFontColor=headerfontcol,
    boxColorOne=boxcolor,
    headershape=roundedright,
    headerfont=\Large\sffamily\bfseries,
    textborder=rectangle,
    background=user,
    headerborder=open,
    boxshade=plain,
    headerheight=3cm
}
%%% Eye Cacther %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
{
}
%%% Title %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
{\sffamily\bfseries
%    \includegraphics[height=3cm]{pix/elte_logo}\
    Can I lower this title?
}
%%% Authors %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
{
    \vspace{1em} Pr. Oblematic\
    {Affiliation \ Long affiliation \ Very long affiliation}
}
%%% Logo %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
{
}
\tikzset{
  remember picture,
  execute at end picture={
%    \tikzset{overlay}% maybe
    \draw[red, ->] (current page.center) -- ++(1cm, 1cm);
  }
}
\headerbox{Problem}{name=problem1,column=0,row=0}{
}
\headerbox{Solution}{name=solution1,column=0,below=problem1,above=bottom}{
}
\headerbox{Problem}{name=problem2,span=2,column=1,row=0}{
}
\headerbox{Solution}{name=solution2,span=2,column=1,below=problem2,above=bottom}{
}
\end{poster}
\end{document}