[This is continuation of the unfinished fix from bug#34517]

>> Stefan, please advise shouldn't selecting a completion from the
>> *Completions* buffer clear the minibuffer's content before
>> inserting the selected completion?
>
> No, for example when you complete file name "C-x C-f ~/.e TAB"  the
> *Completions* buffer will only show ".emacs" so we should clear the
> minibuffer before inserting ".emacs" because that would lose the leading
> "~/".  There are other circumstances where trailing text needs to
> be preserved.

Another example that occurred to me is shell-command M-! minibuffer
where TAB can complete on commands and file names.

> The completion code handles this with `completion-base-position` which
> holds the beginning and end of the text that should be replaced when you
> choose an item in *Completions*.
>
>>> 0. emacs -Q
>>> 1. ‘C-h f TAB’ displays a list of completions
>>> 2. type a nonexistent function name, i.e. some random text
>>>    in the minibuffer, e.g. “blabla”
>
> The *Completions* content is now "out of date" compared to the minibuffer.
>
>>> 3. click on an existing valid completion in the *Completions* buffer,
>>>    e.g. on “append”
>
> completion-base-position was set at step (1) to cover the empty text
> after the prompt, so this empty text (which is now right in front of
> "blabla") is replaced with "append" resulting in "appendblabla".
>
> Obviously, the result is not what we want.
> Now sure how to change which part, tho.  Maybe instead of
> completion-base-position we should store the prefix and suffix strings,
> so when you select an entry from *Completions* we just clear the
> minibuffer and replace it with (concat prefix selection suffix)?

Now I tried this, and it works correctly.  But not sure how to make
this change as backward-compatible as possible.  One variant
would be to save '("prefix" "suffix") instead of '(10 11)
in 'completion-base-position' but this might fail in some existing code.

So maybe better to add a new variable 'completion-base-affixes'.
Then whether to use 'completion-base-position'
or 'completion-base-affixes' could be defined by the new user option
'completion-use-base-affixes'.

Then it can be used in the new command from emacs-devel:

```
(defun minibuffer-completion-choose (&optional arg)
  "Run `choose-completion' from the minibuffer in its scrolling window."
  (interactive "P")
  (with-minibuffer-scroll-window
    (let ((completion-use-base-affixes t))
      (choose-completion nil arg))))
```