Vertical bar appearing except on top of page

Question

I want a command \shybar which prints a \textbar except if it would be the first character on the page. In this case, it should do nothing. Because of this question, I tried the following code:

\documentclass{memoir}

\newcommand*\shybar{\leaders{\mbox\textbar}}

\begin{document}
\shybar This bar should not be printed, but \shybar this one should.
\end{document}

I inserted an \mbox because \leaders asked for a box. Anyway, it did not work; it still asks for boxes. How can I solve this problem?

Answer 1

It occurred to me that \tikz[remember page,overlay] must already wait until the page is laid out in order to work right. Nor will it change the layout since it overlays the existing text.

\documentclass{memoir}
\usepackage{tikzpagenodes}

\newlength{\tempx}
\newlength{\tempy}

\newcommand*\shybar{\ifvmode\tikz[remember picture,overlay]% start of paragraph
{\pgfextracty{\tempy}{\pgfpointdiff{\pgfpointorigin}{\pgfpointanchor{current page text area}{north west}}}%
\ifdim\tempy<\baselineskip\relax%
\else\pgftext[base]{\textbar};\fi}%
\else\tikz[remember picture,overlay]% inside paragraph
{\let\p1=\pgfpointdiff{\pgfpointorigin}{\pgfpointanchor{current page text area}{north west}}%
\pgfextractx{\tempx}{\p1}%
\pgfextracty{\tempy}{\p1}%
\ifdim\tempy<\baselineskip \ifdim\tempx=0pt \relax%
\else\pgftext[base]{\textbar};\fi%
\else\pgftext[base]{\textbar};\fi}\fi%
\hspace*{1pt}}% reserve space

\begin{document}
\shybar This bar should not be printed, but \shybar this one should.

\rule{1pt}{7.1in}

Last line of page\newline
\shybar This bar should not be printed, but \shybar this one should.
\end{document}

This approach has a problem with things like hanging indentation.