[Bug] : Merging border-top and border-color gives unexpected output

[Siilwyn]

Describe the bug
Given a{border-top:1px solid;border-color:purple} results in a{border-color:currentcolor purple purple;border-top:1px solid purple}.

To Reproduce
Run a test like this:

test(
  'keep order of border properties, short- and longhand, right to left',
  passthroughCSS('a{border-top:1px solid;border-color:purple}')
);

Expected behavior
Expected output would I think be the same as the input so: border-top:1px solid;border-color:purple.

Environment
Node.js 14, cssnano on master 4d7fe367.

Additional context
See #1071

[ludofischer]

I think this might be a duplicate of #1044
merge-longhand first splits every shorthand into the single properties, then tries to merge them back, but sometimes it merges back into a form that's longer than the original.

In this case, it goes from border-top: 1px solid;border-color: purple

border-top-width: 1px;
border-top-style: solid;
border-top-color: currentcolor;
border-top-color: purple;
border-left-color: purple;
/* every border-*-color: purple 

Then tries to merge it back and finds that strange result (notice that the new rule seems in fact reasonably equivalent to the original, so at least it's not completely broken)

[alexander-akait]

We really need rewrite our merge on more stable solution, right now it is so complex

[Siilwyn]

Yes, let's close this as a duplicate of #1044.