Use '…' for ellipsis in truncate-string-to-width by default (bug#41250)

* lisp/international/mule-util.el (truncate-string-ellipsis):
Change the default value to nil.
(truncate-string-ellipsis): New function.
(truncate-string-to-width): Use the value returned from the
function 'truncate-string-ellipsis'.

* lisp/tab-bar.el (tab-bar-tab-name-truncated):
* lisp/tab-line.el (tab-line-tab-name-ellipsis):
Take advantage of the improvement of the ellipsis default value
in truncate-string-to-width and truncate-string-ellipsis.

* doc/lispref/display.texi (Size of Displayed Text):
Improve description of truncate-string-ellipsis.

1 files changed, 17 insertions, 4 deletions

diff --git a/lisp/international/mule-util.el b/lisp/international/mule-util.el
index 660ac58e022..d792b2530c4 100644
--- a/lisp/international/mule-util.el
+++ b/lisp/international/mule-util.el
@@ -44,9 +44,22 @@
 	(setq i (1+ i)))))
   string)
 
-(defvar truncate-string-ellipsis "..."  ;"…"
+(defvar truncate-string-ellipsis nil
   "String to use to indicate truncation.
-Serves as default value of ELLIPSIS argument to `truncate-string-to-width'.")
+Serves as default value of ELLIPSIS argument to `truncate-string-to-width'
+returned by the function `truncate-string-ellipsis'.")
+
+(defun truncate-string-ellipsis ()
+  "Return a string to use to indicate truncation.
+Use the value of the variable `truncate-string-ellipsis' when it's non-nil.
+Otherwise, return `…' when it's displayable on the selected frame,
+or `...'.  This function needs to be called on every use of
+`truncate-string-to-width' to decide whether the selected frame
+can display the character `…'."
+  (cond
+   (truncate-string-ellipsis)
+   ((char-displayable-p ?…) "…")
+   ("...")))
 
 ;;;###autoload
 (defun truncate-string-to-width (str end-column
@@ -73,7 +86,7 @@ If ELLIPSIS is non-nil, it should be a string which will replace the
 end of STR (including any padding) if it extends beyond END-COLUMN,
 unless the display width of STR is equal to or less than the display
 width of ELLIPSIS.  If it is non-nil and not a string, then ELLIPSIS
-defaults to `truncate-string-ellipsis'.
+defaults to `truncate-string-ellipsis', or to three dots when it's nil.
 
 If ELLIPSIS-TEXT-PROPERTY is non-nil, a too-long string will not
 be truncated, but instead the elided parts will be covered by a
@@ -81,7 +94,7 @@ be truncated, but instead the elided parts will be covered by a
   (or start-column
       (setq start-column 0))
   (when (and ellipsis (not (stringp ellipsis)))
-    (setq ellipsis truncate-string-ellipsis))
+    (setq ellipsis (truncate-string-ellipsis)))
   (let ((str-len (length str))
 	(str-width (string-width str))
 	(ellipsis-width (if ellipsis (string-width ellipsis) 0))