Improving Rightarrow in tikz-cd

Question

The problem

The tikz/double option which is used by tikz/commuting diagrams/Rightarrow internally creates a wide black line and a slightly less wide white line on top of it to display what looks like a double stroked path (see pgf manual, /tikz/double=). This causes (at least) two types of visual glitches. Notably, several viewers do not display both "strokes" of the path on all zoom levels, as the white part sometimes completely overrides the black path (discussed here, see also the screenshot below). Furthermore, there is a spurious horizontal line at the tail and before the head of the arrow (discussed here):

Question

In a more general sense, the question would be "What is the best way to fix these issues?"

(Originally, there were some specific questions here on an attempted fix. The solution described below implements a more general solution to the problem, rendering the original questions obsolete. See also the discussion on Github for some context.)

Answer 1

I have published my solution on CTAN under the name tikz-nfold. Arbitary TikZ paths and arrows of any order (triple, quadruple, ...) are now supported. Usage is as follows:

\usetikzlibrary{nfold}

[...]

\begin{tikzcd}
  a \ar[r, Rightarrow, bend right, nfold] &
  b \ar[d, Mapsto, nfold=3, "\alpha", "\beta"'] \\
  c \ar[r, Mapsfrom, double distance=4pt, nfold=4] &
  d
\end{tikzcd}