bug#33870: 27.0.50; xref-goto-xref not configurable

[Juri Linkov]

>> For example, I'm not complaining that diff-hl-mode uses non-standard
>> blue color for changes whereas the standard diff-mode color is yellow.
>
> The standard diff-mode color for "change" is "none" (see the diff-changed),
> so I looked at what most other editors use (blue). I've only noticed the
> yellow in diff-refine-changed very recently, and I've yet to see it in
> practice. When is this face used exactly?

It's not used anymore but it was yellow some time ago.  BTW, I still
can't get used to diff-removed yellow and diff-added blue
diff colors on Wikipedia, and they are not configurable.

>> Also I'm not complaining that flymake steals the fringe indicator from
>> diff-hl-mode, although there is no conflict because diff-hl-mode
>> uses the background color whereas flymake uses the foreground color,
>> so they can peacefully coexist together on the fringe.
>
> If only fringes supported that kind of merging. BTW, diff-hl-mode uses the
> foreground color for the border, but any kind of merging would be
> an improvement.

I thought that since add-face-text-property supports face merging
maybe it would be possible to do the same with fringes,
but I have not investigated how feasible it is.