tikz-cd `equal` option produces faint edge across "gap" at ends of path

Question

I want to draw an arrow as an equals sign in a commutative diagram. This should be possible using the equal option provided by tikz-cd. As pointed out in Long equals in tikz, this produces a faint artifact at the ends of the path. There is a small dark edge.

Q: How do I fix this?

I'm stealing the example code from https://tex.stackexchange.com/a/121192/12871:

\documentclass[tikz,convert=false]{standalone}
\usepackage{tikz-cd}
\usetikzlibrary{arrows}
\tikzset{commutative diagrams/.cd,arrow style=tikz,diagrams={>=latex'}}
\begin{document}
\begin{tikzcd}[swap,bend angle=45]
A \dar{f} \rar{a} \ar[bend left,equal]{rr}
  & X \dar{g} \rar{r}
      & A \dar[swap]{f} \\
B \rar[swap]{i}   \ar[bend right,equal]{rr}
      & Y \rar[swap]{\beta}
          & B
\end{tikzcd}
\end{document}

Answer 1

It's known deficit of TikZ. Somewhat annoying but not unavoidable.

Here is a patch. It might not work is all circumstances, so you might want to keep the patch local.

\documentclass[tikz,convert=false]{standalone}

\usepackage{tikz-cd}
    \usetikzlibrary{arrows}
    \tikzset{commutative diagrams/.cd,arrow style=tikz,diagrams={>=latex'}}

\makeatletter
\def\pgf@stroke@inner@line{%
  \let\pgf@temp@save=\pgf@strokecolor@global
  \pgfsys@beginscope%
  {%
    \pgfsys@roundcap% <-- I add this
    \pgfsys@setlinewidth{\pgfinnerlinewidth}%
    \pgfsetstrokecolor{\pgfinnerstrokecolor}%
    \pgfsyssoftpath@invokecurrentpath%
    \pgfsys@stroke%
  }%
  \pgfsys@endscope%
  \global\let\pgf@strokecolor@global=\pgf@temp@save
}

\begin{document}
    \begin{tikzcd}[swap,bend angle=45]
    A \dar{f} \rar{a} \ar[bend left,Rightarrow]{rr}
      & X \dar{g} \rar{r}
          & A \dar[swap]{f} \\
    B \rar[swap]{i}   \ar[bend right,equal]{rr}
          & Y \rar[swap]{\beta}
              & B
    \end{tikzcd}
\end{document}