Clear buffer-undo-list after sending input in ERC

* lisp/erc/erc.el (erc-insert-line): Assume `erc-insert-marker' points
somewhere.
(erc-send-current-line): Set `buffer-undo-list' to nil because it should
only record editing changes in the prompt area, which has just been
cleared.  ERC did this via `erc-display-prompt' prior to 5.6, but it now
leaves the prompt alone by default.
* test/lisp/erc/erc-tests.el (erc-update-undo-list): New test.
(Bug#74518)

lisp/erc/erc.el

@@ -3515,8 +3515,7 @@ modification hooks)."
                     (add-text-properties (point-min) (1+ (point-min)) props)))
                 (erc--refresh-prompt)))))
         (run-hooks 'erc-insert-done-hook)
-        (erc-update-undo-list (- (or (marker-position (or erc--insert-marker
-                                                          erc-insert-marker))
+        (erc-update-undo-list (- (or erc--insert-marker erc-insert-marker
                                      (point-max))
                                  insert-position))))))
 
@@ -8200,10 +8199,9 @@ ERC prints them as a single message joined by newlines.")
                   ;; Fix the buffer if the command didn't kill it
                   (when (buffer-live-p old-buf)
                     (with-current-buffer old-buf
-                      (save-restriction
-                        (widen)
-                        (let ((buffer-modified (buffer-modified-p)))
-                          (set-buffer-modified-p buffer-modified))))))
+                      (setq buffer-undo-list nil)
+                      ;; `set-buffer-modified-p' used to do this here.
+                      (force-mode-line-update))))
 
                 ;; Only when last hook has been run...
                 (run-hook-with-args 'erc-send-completed-hook str)))

test/lisp/erc/erc-tests.el

@@ -2915,6 +2915,85 @@
     (should (equal (erc-tests--format-my-nick "oh my") expect))
     (should (equal (erc--format-speaker-input-message "oh my") expect))))
 
+(ert-deftest erc-update-undo-list ()
+  ;; Remove `stamp' so this can run in any locale.  Alternatively, we
+  ;; could explicitly enable it and bind its format options to strings
+  ;; that lack specifiers (perhaps in a separate test).
+  (let ((erc-modules (remq 'stamp erc-modules))
+        (erc-mode-hook erc-mode-hook)
+        (erc-insert-modify-hook erc-insert-modify-hook)
+        (erc-send-modify-hook erc-send-modify-hook)
+        (inhibit-message noninteractive)
+        marker)
+
+    (erc-stamp-mode -1)
+    (erc-tests-common-make-server-buf)
+    (setq erc-server-current-nick "tester")
+
+    (with-current-buffer (erc--open-target "#chan")
+      ;; Add some filler to simulate more realistic values.
+      (erc-tests-common-simulate-line
+       ":irc.foonet.org 353 tester = #chan :bob tester alice")
+      (erc-tests-common-simulate-line
+       ":irc.foonet.org 366 tester #chan :End of NAMES list")
+      (should (erc-get-server-user "bob"))
+
+      (goto-char (point-max))
+      (should (= (point) 45))
+
+      ;; Populate undo list with contrived values.
+      (let ((kill-ring (list "abc"))
+            interprogram-paste-function)
+        (yank))
+      (push nil buffer-undo-list)
+      (push (point-max) buffer-undo-list)
+      (setq marker (point-marker))
+      (put-text-property 46 47 'face 'warning)
+      (call-interactively #'delete-backward-char 1)
+      (push nil buffer-undo-list)
+      (should (= (point) 47))
+      (should (equal buffer-undo-list `(nil
+                                        ("c" . -47)
+                                        (,marker . -1)
+                                        (nil face nil 46 . 47)
+                                        48
+                                        nil
+                                        (45 . 48))))
+
+      ;; The first char after the prompt is at buffer pos 45.
+      (should (= 40 (- 45 (length (erc-prompt))) erc-insert-marker))
+
+      ;; A new message arrives, growing the buffer by 11 chars.
+      (erc-tests-common-simulate-privmsg "bob" "test")
+      (should (equal (buffer-substring 40 erc-insert-marker) "<bob> test\n"))
+      (should (= (point-max) 58))
+      (should (= 11 (length "<bob> test\n") (- (point) 47)))
+
+      ;; The list remains unchanged relative to the end of the buffer.
+      (should (equal buffer-undo-list `(nil
+                                        ("c" . -58)
+                                        (,marker . -1)
+                                        (nil face nil 57 . 58)
+                                        59
+                                        nil
+                                        (56 . 59))))
+
+      ;; Undo behavior works as expected.
+      (undo nil)
+      (should (erc-tests-common-equal-with-props
+               (buffer-substring erc-input-marker (point-max))
+               #("abc" 1 2 (face nil))))
+      (should (equal (take 4 buffer-undo-list)
+                     `((nil face warning 57 . 58)
+                       (58 . 59)
+                       nil
+                       ("c" . -58))))
+      (undo 2)
+      (should (string-empty-p (erc-user-input)))))
+
+  (when noninteractive
+    (erc-tests-common-kill-buffers)))
+
 (ert-deftest erc--route-insertion ()
   (erc-tests-common-prep-for-insertion)
   (erc-tests-common-init-server-proc "sleep" "1")