bug#33992: 27.0.50; xref-find-definitions wastes too much space

[Juri Linkov <juri@linkov.net>]

>>> Does it feel the same way to you?
>>
>> The difference is that completions pop up in a small unobtrusive window.
>
> Small window? I usually have a side-by-side fullscreen split, and if
> I initiate completion in one of the windows, *Completion* takes up the
> whole other window. Temporarily, of course.

The key word here is 'Temporarily'.  Unlike *Completions*,
the *xref* buffer doesn't go out easily.

>>    (defun display-buffer-condition-xref (buffer-name _action)
>>      (and (string-match-p "\\`\\*\\(xref\\)\\*\\(\\|<[0-9]+>\\)\\'"
>>                           buffer-name)
>>           (memq this-command '(xref-find-definitions))))
>>
>>    (defun display-buffer-condition-from-xref (_buffer-name _action)
>>      (string-match-p "\\`\\*\\(xref\\)\\*\\(\\|<[0-9]+>\\)\\'"
>>                      (buffer-name (current-buffer))))
>
> This function seems unused.

It's unused because it would be useful only in the *xref* buffer
created by the xref-find-definitions command, so xref needs to
provide a way to distinguish such case.

>>    (setq display-buffer-alist
>>       '((display-buffer-condition-xref
>>          display-buffer-in-direction
>
> And this function is undefined in my Emacs.

This function is implemented by Martin in bug#33870.

>>    (with-eval-after-load 'xref
>>      (define-key xref--button-map [(control ?m)] #'xref-quit-and-goto-xref))
>>
>> How do you like that?
>
> I might, but since I can't really try your customization myself yet, I'll
> repeat a question you might be familiar with already:
>
>   Will this also affect xref-find-references and project-find-regexp?

It should not affect them due to (memq this-command '(xref-find-definitions))
above.  But also to not affect commands active in the *xref* buffer,
xref should provide a way to check if the *xref* buffer was created
by xref-find-definitions.