Let org-columns correctly detect string-widths in code

TODO: maybe I should also make a test directly on
`org-columns-add-ellipses'.  Will do in next iteration unless
objections.

* lisp/org-colview.el (org-columns--truncate-below-width): add a
helper function that will trim off just enough data from string to
fit into expected width.
(org-columns-add-ellipses): make sure to do truncation correctly
even in CJK locales (where an ellipsis character takes two
spaces).

* testing/lisp/test-org-colview.el
(test-org-colview/substring-below-width): add test to make sure
helper function is correct.
(test-org-colview/columns-width): fix incorrect expectations for
CJK locales about ellipses.

lisp/org-colview.el

@@ -452,13 +452,29 @@ DATELINE is non-nil when the face used should be
 	    "Type \\<org-columns-map>`\\[org-columns-edit-value]' \
 to edit property")))))))
 
+(defun org-columns--truncate-below-width (string width)
+  "Return a substring of STRING no wider than WIDTH.
+This substring must start at 0, and must be the longest possible
+substring whose `string-width' does not exceed WIDTH."
+  (declare (side-effect-free t))
+  (let ((end (min width (length string))) res)
+    (while (and end (>= end 0))
+      (let* ((curr (string-width string 0 end))
+             (excess (- curr width)))
+        (if (> excess 0)
+            (cl-decf end (max 1 (/ excess 2)))
+          (setq res (substring string 0 end) end nil))))
+    res))
+
 (defun org-columns-add-ellipses (string width)
   "Truncate STRING with WIDTH characters, with ellipses."
   (cond
-   ((<= (length string) width) string)
+   ((<= (string-width string) width) string)
-   ((<= width (length org-columns-ellipses))
+   ((<= width (string-width org-columns-ellipses))
-    (substring org-columns-ellipses 0 width))
+    (org-columns--truncate-below-width org-columns-ellipses width))
-   (t (concat (substring string 0 (- width (length org-columns-ellipses)))
+   (t (concat
+       (org-columns--truncate-below-width
+        string (- width (string-width org-columns-ellipses)))
        org-columns-ellipses))))
 
 (defvar org-columns-full-header-line-format nil

testing/lisp/test-org-colview.el

@@ -92,6 +92,23 @@
           (org-columns-compile-format
            "%ITEM{+;%.1f}"))))
 
+(ert-deftest test-org-colview/substring-below-width ()
+  "Test `org-columns--truncate-below-width'."
+  (cl-flet ((check (string width expect)
+              (string= expect (org-columns--truncate-below-width
+                               string width))))
+    (if (= (char-width ?…) 2)
+        (progn (should (check "12…" 3 "12"))
+               (should (check "1…2" 1 "1"))
+               (should (check "1…2" 2 "1"))
+               (should (check "1…2" 3 "1…"))
+               (should (check "……………………" 7 "………")))
+      (progn (should (check "12…" 4 "12…"))
+             (should (check "1…2" 1 "1"))
+             (should (check "1…2" 2 "1…"))
+             (should (check "1…2" 3 "1…2"))
+             (should (check "……………………" 7 "…………………"))))))
+
 (ert-deftest test-org-colview/get-format ()
   "Test `org-columns-get-format' specifications."
   ;; Without any clue, use `org-columns-default-format'.
@@ -226,7 +243,7 @@
 	      (org-columns))
 	    (org-trim (get-char-property (point) 'display)))))
   (should
-   (equal "1234… |"
+   (equal (if (= 1 (char-width ?…)) "1234… |" "123… |")
 	  (org-test-with-temp-text "* H\n:PROPERTIES:\n:P: 123456\n:END:"
 	    (let ((org-columns-default-format "%5P")
 		  (org-columns-ellipses "…"))