master 1f63331: * lisp/subr.el (activate-change-group): Refine fix for bug#33341

[Stefan Monnier]

branch: master
commit 1f633311c194edff9976b0672940769e76ac538e
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Commit: Stefan Monnier <monnier@iro.umontreal.ca>

    * lisp/subr.el (activate-change-group): Refine fix for bug#33341
---
 lisp/subr.el            | 15 +++++++++++++--
 test/lisp/subr-tests.el | 15 +++++++++++++--
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/lisp/subr.el b/lisp/subr.el
index 1cf3a49..0b92a4f 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3038,8 +3038,19 @@ to `accept-change-group' or `cancel-change-group'."
       (if (eq buffer-undo-list t)
          (setq buffer-undo-list nil)
        ;; Add a boundary to make sure the upcoming changes won't be
-       ;; merged with any previous changes (bug#33341).
-       (undo-boundary)))))
+       ;; merged/combined with any previous changes (bug#33341).
+       ;; We're not supposed to introduce a real (visible)
+        ;; `undo-boundary', tho, so we have to push something else
+        ;; that acts like a boundary w.r.t preventing merges while
+       ;; being harmless.
+        ;; We use for that an "empty insertion", but in order to be harmless,
+        ;; it has to be at a harmless position.  Currently only
+        ;; insertions are ever merged/combined, so we use such a "boundary"
+        ;; only when the last change was an insertion and we use the position
+        ;; of the last insertion.
+        (when (numberp (caar buffer-undo-list))
+          (push (cons (caar buffer-undo-list) (caar buffer-undo-list))
+                buffer-undo-list))))))
 
 (defun accept-change-group (handle)
   "Finish a change group made with `prepare-change-group' (which see).
diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index e3f798d..019441d 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -555,13 +555,24 @@ See https://debbugs.gnu.org/cgi/bugreport.cgi?bug=19350.";
   (with-temp-buffer
     (buffer-enable-undo)
     (insert "0\n")
-    ;; (undo-boundary)
     (let ((g (prepare-change-group)))
       (activate-change-group g)
       (insert "b\n")
       (insert "c\n")
       (cancel-change-group g))
-    (should (equal (buffer-string) "0\n"))))
+    (should (equal (buffer-string) "0\n"))
+    (erase-buffer)
+    (setq buffer-undo-list nil)
+    (insert "0\n")
+    (let ((g (prepare-change-group)))
+      (activate-change-group g)
+      (insert "b\n")
+      (insert "c\n")
+      (accept-change-group g))
+    (should (equal (buffer-string) "0\nb\nc\n"))
+    (undo-boundary)
+    (undo)
+    (should (equal (buffer-string) ""))))
 
 (provide 'subr-tests)
 ;;; subr-tests.el ends here