New defcustom 'tooltip-resize-echo-area'

* lisp/tooltip.el (tooltip-resize-echo-area): New defcustom.
(tooltip-show-help-non-mode): Use it to avoid truncating the
tooltip text in the echo area.  (Bug#28724)

* etc/NEWS: Mention 'tooltip-resize-echo-area'.

etc/NEWS

@@ -31,6 +31,12 @@ When you add a new item, use the appropriate mark if you are sure it applies,
 
 * Changes in Emacs 27.1
 
+---
+** The new option 'tooltip-resize-echo-area' avoids truncating tooltip text
+on GUI frames when tooltips are displayed in the echo area.  Instead,
+it resizes the echo area as needed to accommodate the full tool-tip
+text.
+
 +++
 ** New function 'logcount' calculates an integer's Hamming weight.
 

lisp/tooltip.el

@@ -155,6 +155,18 @@ This variable is obsolete; instead of setting it to t, disable
 (make-obsolete-variable 'tooltip-use-echo-area
 			"disable Tooltip mode instead" "24.1" 'set)
 
+(defcustom tooltip-resize-echo-area nil
+  "If non-nil, using the echo area for tooltips will resize the echo area.
+By default, when the echo area is used for displaying tooltips,
+the tooltip text is truncated if it exceeds a single screen line.
+When this variable is non-nil, the text is not truncated; instead,
+the echo area is resized as needed to accommodate the full text
+of the tooltip.
+This variable has effect only on GUI frames."
+  :type 'boolean
+  :group 'tooltip
+  :version "27.1")
+
 
 ;;; Variables that are not customizable.
 
@@ -347,7 +359,8 @@ It is also called if Tooltip mode is on, for text-only displays."
 						   (current-message))))
         (setq tooltip-previous-message (current-message)))
       (setq tooltip-help-message help)
-      (let ((message-truncate-lines t)
+      (let ((message-truncate-lines
+             (or (not (display-graphic-p)) (not tooltip-resize-echo-area)))
             (message-log-max nil))
         (message "%s" help)))
      ((stringp tooltip-previous-message)