bug#38645: 26.3; minibuffer input is called with multi-line window when multi-line message is shown

[Eli Zaretskii <eliz@gnu.org>]

> From: ynyaaa@gmail.com
> Cc: 38645@debbugs.gnu.org
> Date: Fri, 20 Dec 2019 11:16:31 +0900
> 
> With (setq resize-mini-windows t), the key sequence below shows message
> in one-line window. Then input motion commands and the minibuffer is
> redisplayed in one-line window. Eval: prompt is hidden.
> 
> M-:
> C-q C-j
> ?a
> C-x C-e

Martin, what do you think about the following fix?  Is it correct, and
if so, safe enough for the release branch?

diff --git a/src/xdisp.c b/src/xdisp.c
index 3080f8920a..438267350f 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -11562,7 +11562,15 @@ resize_mini_window (struct window *w, bool exact_p)
 
       SET_MARKER_FROM_TEXT_POS (w->start, start);
 
-      if (EQ (Vresize_mini_windows, Qgrow_only))
+      if (EQ (Vresize_mini_windows, Qgrow_only)
+	  /* Don't automatically shrink the mini-window, even if
+	     resize-mini-windows is t, if we are displaying an
+	     echo-area message while the minibuffer is active.  */
+	  || (minibuf_level > 0
+	      && WINDOW_LIVE_P (minibuf_window)
+	      && XWINDOW (minibuf_window) == w
+	      && !NILP (echo_area_buffer[0])
+	      && EQ (echo_area_window, minibuf_window)))
 	{
 	  /* Let it grow only, until we display an empty message, in which
 	     case the window shrinks again.  */