Answer by samcarter_is_at_topanswers.xyz for Two minipage with horizontal lines and a vertical line

I'd suggest some kind of table. Here an example with tabularray:

\documentclass{article}\usepackage{xcolor}\usepackage{tabularray}\newcommand{\foo}[2]{%  \begin{tblr}{    colspec={Q[.2\textwidth,halign=r]Q[.8\textwidth-24.4pt]},    column{1}={fg=blue},    hlines,    vline{2}={solid}  }  #1 & #2\\  \end{tblr}%}\begin{document}\noindent\foo{%  blue blue blue blue blue}{%  \[a=b\]  black black black black black black black black black black black black black black black black black black black black black black}%\end{document}