Test if tikzmark exists on same page

Question

Apologies for flooding the site with tikzmark problems ... :)

I know that with \iftikzmark I can test if a given tikzmark exists in general. Is there a way to test if a particular tikzmark exists on the same page?

Use case: I'm trying to design macros that I can insert anywhere in a text that would draw a line between them –- a \linestart and a \linefinish command. However, when the \linefinish command happens to fall on a subsequent page, the line gets drawn to where the \linestart command was on the previous page. I would rather have it instead "do something else", here, just for purpose of illustration, draw me a black square – or, ideally "point across" the page in the direction to where the other mark is sitting.

In the code below, I've tried to construct what I'm envisioning using \iftikzmark but that does not work.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\newcounter{tikzmkstart}\setcounter{tikzmkstart}{0} % 
\newcounter{tikzmkfinish}\setcounter{tikzmkfinish}{0} % 

\newcommand{\linestart}{%
\stepcounter{tikzmkfinish}\iftikzmark{b\thetikzmkfinish}{% tikzmark exists on same page
\stepcounter{tikzmkstart}\tikzmarknode{a\thetikzmkstart}{\vphantom{x}}\addtocounter{tikzmkfinish}{-1}}
{% tikzmark doesn't exist on page
\begin{tikzpicture} \draw [line width=4] (0,0) -- (.1,0); \end{tikzpicture}\addtocounter{tikzmkfinish}{-1}}}% black square if linefinish on next page

\newcommand{\linefinish}{%
\iftikzmark{a\thetikzmkstart}{% tikzmark exists on same page
\stepcounter{tikzmkfinish}\tikzmarknode{b\thetikzmkfinish}{\vphantom{x}}\begin{tikzpicture}[remember picture]  \draw[overlay] (a\thetikzmkstart.west) -- (b\thetikzmkfinish.west); \end{tikzpicture}}
{% tikzmark doesn't exist on page
\begin{tikzpicture} \draw [line width=4] (0,0) -- (.1,0); \end{tikzpicture}}}

\begin{document} 
Text \linestart text text 

Text text text 
%\newpage

Text text \linefinish text
\end{document}

Any ideas of how else I might accomplish something like this would be very welcome!

---

Update: Based on @marmot's suggestion to look at this answer, I've now come up with the following. However, I'm stuck with how to make the lines "point across" the page breaks ...

\documentclass{article}
\usepackage{refcount}
\usepackage{tikz}
\usetikzlibrary{calc}

\newcounter{tmp}
\newcommand\tikzmark[1]{%
  \tikz[overlay,remember picture] \node (#1) {};}

\newcommand\linestart{%
  \stepcounter{tmp}%
  \tikzmark{a}\label{a\thetmp}%
  \ifnum\getpagerefnumber{a\thetmp}=\getpagerefnumber{b\thetmp} \else
  \begin{tikzpicture}[overlay, remember picture]
    \draw [thick]     
      (a.west) -- (b.west); 
  \end{tikzpicture}%
  \fi%
}


\newcommand\linefinish{%
\tikzmark{b}\label{b\thetmp}
  \ifnum\getpagerefnumber{a\thetmp}=\getpagerefnumber{b\thetmp}
  \begin{tikzpicture}[overlay, remember picture]
    \draw [thick]
      (a.west) -- (b.west); 
  \end{tikzpicture}%
  \else
  \begin{tikzpicture}[overlay, remember picture]
    \draw [thick]
      (a.west) -- (b.west); 
  \end{tikzpicture}%
  \fi
}

\newcommand\Squ[1]{\linestart#1\linefinish}

\begin{document}

Text \linestart text text

Text text text

Text text \linefinish text

Text \linestart text text

Text text text
\newpage

Text text \linefinish text

\end{document}

Answer 1

I can see the point of having a \iftikzmarkonpage conditional, so I've added it to the tikzmark package (download tikzmark.dtx and run tex tikzmark.dtx to get the latest version). This introduces a new conditional, \iftikzmarkoncurrentpage{<tikzmark name>}. That copes with the issue about the lines being drawn on both pages when needed.

\documentclass{article}
\usepackage[a6paper]{geometry}
%\url{https://tex.stackexchange.com/q/483547/86}
\usepackage{tikz}
\usepackage{tikzpagenodes}
\usetikzlibrary{tikzmark}
\newcounter{tikzmarklines}\setcounter{tikzmarklines}{0} % 

\tikzset{
  tikzmark prefix=prefix-,
  tikzmark suffix=-suffix
}

\newcommand{\linestart}{%
  \stepcounter{tikzmarklines}%
  \tikzmark{a\thetikzmarklines}%
  \iftikzmarkoncurrentpage{b\thetikzmarklines}%
  \else
  \begin{tikzpicture}[remember picture,overlay,next page=below]%
  \clip (current page text area.south west) rectangle (current page text area.north east);
  \draw[line width=4] (pic cs:a\thetikzmarklines) -- (pic cs:b\thetikzmarklines);
  \end{tikzpicture}%
  \fi
}

\newcommand{\linefinish}{%
  \tikzmark{b\thetikzmarklines}%
  \begin{tikzpicture}[remember picture,overlay,next page=below]%
  \clip (current page text area.south west) rectangle (current page text area.north east);
  \draw[line width=1] (pic cs:a\thetikzmarklines) -- (pic cs:b\thetikzmarklines);
  \end{tikzpicture}%
  }

\begin{document} 
Text \linestart text text 

Text text text 

Text text \linefinish text

\newpage

Text text text 

Text \linestart text text 

Text text text 
\newpage

Text text text

Text text \linefinish text
\end{document}