bug#40919: 27.0.91; next-error-select-buffer does not always behave as documented

[Juri Linkov]

>> Another variant would be
>> to add an optional choice to the existing option that allows to restore
>> the old behavior and doesn't affect the current behavior in any way.
>
> FWIW, I'd consider that more of a documentation change.
>
> More importantly, and certainly only for Emacs 28, Juri could you remind me
> what we'll be losing by removing the case no. 2 from
> next-error-find-buffer?

It is used to handle such cases when navigating one next-error list
visits a buffer that contains another next-error list - visiting
such buffer should not switch the navigation, it should continue
the initial navigation.

But I'm not proud of the case no. 2, so you can drop it :)

Or better move it to a separate function and provide this function
as an option in next-error-find-buffer-function.

> The ability to switch to an arbitrary Grep buffer and start using it with
> 'M-x next-error'? E.g. if there are several of them. That's more of
> a backward compatibility concern, right? Or do you have scenarios in
> mind where this will really save on keystrokes?

The ability to visit an arbitrary Grep buffer and use 'next-error' in it
to switch navigation should always work as the most reliable and implicit
way to switch navigation.  This is the case no. 3 in next-error-find-buffer.

> On another note, perhaps we could add a message to next-error-select-buffer
> that would be shown if we suspect this command will not have the expected
> result for the user.

Or maybe ask the user using the minibuffer to resolve ambiguity.