Exiting completion-in-region-mode (was: Handle case where `beg` and `end` are strings instead of markers)

[Juri Linkov]

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.