bug#35737: xref--original-command

[Juri Linkov]

>> Remembering the command that created the *xref* buffer
>> will allow such customization to treat RET differently,
>> i.e. it makes sense for RET to quit the transient xref buffer
>> displayed momentarily while navigating code with xref-find-definitions,
>> whereas leaving the xref buffer visible while navigating matches
>> in the xref buffer created by e.g. project-find-regexp:
>>
>> (define-key xref--button-map [(control ?m)]
>>    (lambda ()
>>      (interactive)
>>      (if (memq xref--original-command '(xref-find-definitions))
>>          (call-interactively 'xref-quit-and-goto-xref)
>>        (call-interactively 'xref-goto-xref))))
>
> I'm all for customizability, but as I said in a past discussion on the
> subject, I don't think this by itself would be a good default behavior.

I don't propose to change its default behavior.  Just allowing its
customization would be good.

> Having two buffers that looks basically the same but react to RET
> differently is not great for usability.

Using display-buffer-in-direction, they don't look the same: after
customization xref-find-definitions can display the xref buffer below,
whereas other xref command can display in the side window.

>> Better ideas?
>
> Ideally, the "transient" buffer should look differently from the
> "persistent" one.
>
> To take VS Code as an example (I just to see how it behaves exactly),
> a search for references opens search results in the sidebar.

Emacs has side windows too.

> Whereas for definitions they have both "Go To Definition" and "Peek
> Definition". The latter shows the definitions kind of inline, in
> a not-exactly-a-popup kind of way
> (https://stackoverflow.com/questions/55599177/go-to-definition-in-vscode-preview-window-ruins-the-edito),
> and there, if you type RET, that triggers navigation to the selected
> definition. "Go To Definition", when there are several definitions, also
> switches to "Peek" mode by default.
>
> Not sure how best to implement this different kind of display in Emacs, but
> the idea makes a lot of sense to me.
>
> In Sublime, IIRC, for this they used the dropdown from their
> top-of-the-window command line, the same one that becomes active once one
> presses Ctrl-P.

We have Completions for the same purpose.