bug#46718: 27.1; truncate-lines in minibuffer prevents auto resize

[Eli Zaretskii <eliz@gnu.org>]

> From: Clemens <clemera@posteo.net>
> Date: Tue, 23 Feb 2021 14:44:20 +0100
> 
> ;; emacs -Q
> (setq resize-mini-windows t)
> (minibuffer-with-setup-hook
>      (lambda ()
>        (setq-local truncate-lines t)
>        (let ((ov (make-overlay (point-max) (point-max)))
> 	    (minibuf-after-string " \nOne\nTwo\nThree"))
> 	(put-text-property 0 1 'cursor t minibuf-after-string)
> 	(overlay-put ov 'after-string minibuf-after-string)))
>    (read-string ":"))
> 
> the mini window will not resize but when commenting out the setting for
> truncate-lines it resizes appropriately.

This is (was) not supported.  When truncate-lines is non-nil in the
minibuffer, Emacs assumed the minibuffer text is just one line.  And I
can understand that assumption: frankly, setting truncate-lines with
multi-line text in the minibuffer makes little sense, because it means
some of the text will not be shown, something that contradicts the
very purpose of resizing the mini-window.  Why would someone do
something weird like that?

Anyway, I've made this work as expected on the master branch now.