Re: BIKESHED: completion faces

[Dmitry Gutov]

On 27.10.2019 3:58, Stefan Monnier wrote:
> In Emacs-26, completion faces looked like the following:
>
> - the "common" part shown like the default face.
> - the "first-difference" shown in bold.
> - everything else uses the default face.
>
> The "common" part is the part applied to the characters that are also
> found in the current minibuffer.  E.g. when completing
>
>     M-x ne-li ?
>
> the "ne" and "-li" chars in the *Completions* are the "common" part.
>
> With the basic, prefix completion, the "common" part is not very
> important and is separated from the rest at the "first-difference", so
> the default faces make a fair bit of sense there.
>
> But for other completion styles such as `substring`,
> `partial-completion`, and even more so for `flex`, it's not always
> immediately obvious how the minibuffer contents relate to the possible
> completions displayed in *Completions*.

I wonder if it's *that* important, to be able to see the matching chars.

The -first-difference face shows actionable info: the next character to 
type, to narrow down the completions list as much as possible while 
ensuring that a given completion is still in. You could type other 
chars, but the likelihood of ending up with several options (and no more 
characters to type) will be higher.

So IMHO we could do nothing, and it will be a valid choice. As long as 
all completion styles highlight completion-first-difference properly.

Furthermore, the position of -first-difference also hints at what 
characters were matched (definitely ones before it).

> For this reason, it is common in other completion systems to
> highlight the "common" part somehow (e.g. underline, bold, ...).

Personally, I'm not a big fan of underline, italic and even bold (unless 
it's used sparingly, like it is now). With that said...

> I think Emacs's defaults should be changed so that the "common" part
> (which uses the `completions-common-part` face) is highlighted somehow.
>
> So, what should we go for:
> - bold (but then we need to change the `completions-first-difference`
>    face to keep it different, e.g. underlined)?

If we end up doing this, we might as well change 
completions-first-difference to be indistinguishable from 'default'. 
Because the common part, when highlighted, duplicates the information. 
The "first difference" character is simply the one after the last 
segment of the common part, isn't it?

> - underlined (tho I don't like underlined text very much)?
> - some foreground color?
> - some background color?
> - something else?

If we do end up highlighting completions-common-part somehow, I vote for 
background color, some very subtle one. Because using bold is going to 
be annoying with prefix-based styles, and I don't think we can/should 
use different methods for different styles.

Using "white smoke" seems okay with the default theme, as well as with 
the light-background theme I'm using. No idea what color would be good 
with dark-background themes.