bug#60015: 29.0.60; multi-line messages truncated in non-selected minibuffer

[Juri Linkov]

0. emacs -Q
1. (run-with-timer 5 5 'message "abc\ndef")
2. then activate the minibuffer and switch back to the original buffer
3. after some time observe that only the first line of the multi-line message
   is displayed in the minibuffer

I don't know where this problem comes from, but it's impossible to use
edebug to debug minibuffer-related bugs while the minibuffer is active,
because only an old message is displayed, not the most recent,
when using set-multi-message to collect messages.  So here is
at least the fix for set-multi-message:

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index cfdf2b43f42..36553eca1ad 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -914,6 +914,11 @@ set-multi-message
   "Return recent messages as one string to display in the echo area.
 Note that this feature works best only when `resize-mini-windows'
 is at its default value `grow-only'."
+  (if (and (active-minibuffer-window) (not (minibufferp)))
+      ;; Multi-line messages are limited to one line only
+      ;; when the minibuffer window is active and not selected.
+      ;; So don't return a multi-line string in this case:
+      message
     (let ((last-message (car multi-message-list)))
       (unless (and last-message (equal message (aref last-message 1)))
         (when last-message
@@ -931,7 +936,7 @@ set-multi-message
           (setf (nthcdr multi-message-max multi-message-list) nil)))
       (mapconcat (lambda (m) (aref m 1))
                  (reverse multi-message-list)
-               multi-message-separator)))
+                 multi-message-separator))))
 
 (defun clear-minibuffer-message ()
   "Clear minibuffer message.