Re: Exiting completion-in-region-mode

[Juri Linkov <juri@linkov.net>]

> BTW, there is a related problem: while using `M-down` in the minibuffer,
> it unexpectedly closes the *Completions* window when using
> completion-in-region mode in the minibuffer, such as with
> `M-: (c TAB M-down` or `M-! c TAB M-down`.
>
> The code responsible for automatic closing the *Completions* window
> is found in `completion-at-point`:
>
>               (completion-in-region-mode-predicate
>                (lambda ()
>                  ;; We're still in the same completion field.
>                  (let ((newstart (car-safe (funcall hookfun))))
>                    (and newstart (= newstart start)))))
>
> And I'm completely lost how to adjust it to not exit completion-in-region-mode.

I realized now that inserting each candidate to the ordinary buffer
while navigating the list of in-buffer completions not only difficult to
implement, but also causes a lot of usability problems such as adding
a new entry to the undo list for every M-down/M-up.

OTOH, when forcing the value minibuffer-completion-auto-choose to nil
for in-buffer completions, it works surprisingly well: it navigates
the completions list without inserting candidates to the buffer,
then M-RET inserts the selected completion.

All this is achievable with a small patch:

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index f3003505f1..dfa7640ebe 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -2624,7 +2624,10 @@ completion-in-region-mode-map
   ;; FIXME: Only works if completion-in-region-mode was activated via
   ;; completion-at-point called directly.
   "M-?" #'completion-help-at-point
-  "TAB" #'completion-at-point)
+  "TAB" #'completion-at-point
+  "M-<up>"    #'minibuffer-previous-completion
+  "M-<down>"  #'minibuffer-next-completion
+  "M-RET"     #'minibuffer-choose-completion)
 
 ;; It is difficult to know when to exit completion-in-region-mode (i.e. hide
 ;; the *Completions*).  Here's how previous packages did it:
@@ -2671,6 +2674,7 @@ completion-in-region-mode
     (cl-assert completion-in-region-mode-predicate)
     (setq completion-in-region-mode--predicate
 	  completion-in-region-mode-predicate)
+    (setq-local minibuffer-completion-auto-choose nil)
     (add-hook 'post-command-hook #'completion-in-region--postch)
     (push `(completion-in-region-mode . ,completion-in-region-mode-map)
           minor-mode-overriding-map-alist)))