bug#49844: Highlight non-selected minibuffer prompt

[Juri Linkov]

>>> I like it -- but does this need to be tied to
>>> minibuffer-depth-indicate-mode?  Indicating that the minibuffer isn't
>>> active in this way may be attractive even if you don't have that
>>> switched on?
>>
>> Yeah, I don't like tying to minibuffer-depth-indicate-mode too.
>> But I have no idea where to put it otherwise.
>> Adding a new package for 25 lines makes no sense.
>
> Can't we just put it in minibuffer.el?  (But then there should probably
> be a user option to switch it on/off.)

Ironically, minibuffer.el is intended for code that deals with
completions.  Its first line is

  ;;; minibuffer.el --- Minibuffer completion functions

But most code that deals with the minibuffer
is somewhere in the middle of simple.el.

So neither minibuffer.el nor simple.el is suitable.
Perhaps this was the reason why there are a lot of tiny
packages that provide various minibuffer features,
such as mb-depth.el, minibuf-eldef.el, rfn-eshadow.el, etc.

Maybe it would make sense to create a new file with the name e.g.
minibuf-x.el as a mixed bag of tiny minibuffer features?