delsel.el: Improve the overlay for text replacement

* lisp/delsel.el (delete-selection-replacement-face): Delete var.
(delete-selection-replacement): New face, to replace it.
(delsel--replace-overlay, delsel--replace-text): Delete vars and ...
(delete-selection--replacement-text): ...use this single var instead.
(delete-selection--replacement-cursor): New function.
(delete-active-region): Use it with `cursor-sensor-mode` to avoid the
overlay lingering too long.
(delete-selection--replacement-text): New function extracted from
`delete-selection-repeat-replace-region`, with adjustments to account
for the above changes.
(delete-selection-repeat-replace-region): Use it.

* lisp/emacs-lisp/cursor-sensor.el (cursor-sensor--detect): Do use
`get-pos-property` when it works.  Fix the `missing-p` subfunction
so as not to get fooled by a missing property at END.

etc/NEWS

@@ -995,6 +995,11 @@ of the modified region.
 
 * Changes in Specialized Modes and Packages in Emacs 31.1
 
+** Delete-selection mode
+*** New face 'delete-selection-replacement' for the replacement text
+This comes with a change to how we track what is considered "the
+replacement text", which should be more robust now, and is made
+more clear by the highlighting.
 ** Editorconfig
 
 ---

lisp/delsel.el

@@ -68,16 +68,12 @@ Value must be the register (key) to use."
           (character :tag "Register (Key)"))
   :group 'editing-basics)
 
-(defcustom delete-selection-replacement-face 'highlight
+(defface delete-selection-replacement
-  "If non-nil, active region replacement text is shown in this face.
+  '((t :inherit highlight))
+  "Show the active region replacement text in this face.
 The highlighted text is the text that will be inserted by
 the `delete-selection-repeat-replace-region' command."
-  :type 'face
+  :group 'editing-basics)
-  :group 'editing-basics
-  :set (lambda (symbol value)
-         (set-default symbol value)
-         (if delsel--replace-overlay
-             (overlay-put delsel--replace-overlay 'face value))))
 
 (defcustom delete-selection-temporary-region nil
   "Whether to delete only temporary regions.
@@ -128,8 +124,8 @@ For compatibility with features and packages that are aware of
     (setq-default delete-selection-mode nil) ; But keep it globally disabled.
     )))
 
-(defvar delsel--replace-overlay nil)    ;overlay
+(defvar delete-selection--replacement-text nil
-(defvar delsel--replace-text nil)    ;text from overlay
+  "Can be a string or an overlay.")
 
 ;;;###autoload
 (defun delete-active-region (&optional killp)
@@ -145,17 +141,50 @@ the active region is killed instead of deleted."
    (delete-selection-save-to-register
     (set-register delete-selection-save-to-register
                   (funcall region-extract-function t))
-    (if delsel--replace-overlay
+    (if (overlayp delete-selection--replacement-text)
-        (move-overlay delsel--replace-overlay (point) (point) (current-buffer))
+        (move-overlay delete-selection--replacement-text
-      (setq delsel--replace-overlay
+                      (point) (point) (current-buffer))
-            (make-overlay (point) (point) (current-buffer) nil t))
+      (setq delete-selection--replacement-text
-      (if delete-selection-replacement-face
+            (make-overlay (point) (point) nil nil t))
-          (overlay-put delsel--replace-overlay 'face
+      (overlay-put delete-selection--replacement-text 'face
-                       delete-selection-replacement-face)))
+                   'delete-selection-replacement))
-    (setq delsel--replace-text nil))
+      ;; Make sure the overlay doesn't linger indefinitely.
+      (overlay-put delete-selection--replacement-text
+                   'cursor-sensor-functions
+                   (list #'delete-selection--replacement-cursor))
+      (unless (bound-and-true-p cursor-sensor-mode) (cursor-sensor-mode 1)))
    (t
     (funcall region-extract-function 'delete-only))))
 
+(defun delete-selection--replacement-cursor (_window _oldpos dir)
+  (when (and (overlayp delete-selection--replacement-text)
+             (eq dir 'left))
+    ;; The replacement is considered done: Delete the overlay and store
+    ;; its contents.
+    ;; FIXME: Maybe we should briefly flash the highlighting?
+    (delete-selection--replacement-text)))
+
+(defun delete-selection--replacement-text ()
+  ;; If this is the first use after overwriting regions,
+  ;; find the replacement text by looking at the overlay.
+  (when (overlayp delete-selection--replacement-text)
+    (if (null (overlay-buffer delete-selection--replacement-text))
+        (setq delete-selection--replacement-text nil)
+      (with-current-buffer (overlay-buffer delete-selection--replacement-text)
+        (let ((s (overlay-start delete-selection--replacement-text))
+              (e (overlay-end delete-selection--replacement-text)))
+          (delete-overlay delete-selection--replacement-text)
+          (if (= s e)
+              (setq delete-selection--replacement-text nil)
+            (setq delete-selection--replacement-text
+                  (filter-buffer-substring s e))
+            (set-text-properties
+             0 (length delete-selection--replacement-text)
+             nil delete-selection--replacement-text))))))
+  (cl-assert (or (null delete-selection--replacement-text)
+                 (stringp delete-selection--replacement-text)))
+  delete-selection--replacement-text)
+
 (defun delete-selection-repeat-replace-region (arg)
   "Repeat replacing text of highlighted region with typed text.
 Search for the next stretch of text identical to the region last replaced
@@ -169,28 +198,14 @@ Just `\\[universal-argument]' means repeat until the end of the buffer's accessi
                  (prefix-numeric-value current-prefix-arg))))
     (if (not (and old-text (> (length old-text) 0)))
         (message "No known previous replacement")
-      ;; If this is the first use after overwriting regions,
+      (let ((string (delete-selection--replacement-text)))
-      ;; find the replacement text by looking at the undo list.
+        (if string
-      (when (and (null delsel--replace-text)
-                 delsel--replace-overlay
-                 (buffer-live-p (overlay-buffer delsel--replace-overlay)))
-        (with-current-buffer (overlay-buffer delsel--replace-overlay)
-          (let ((s (overlay-start delsel--replace-overlay))
-                (e (overlay-end delsel--replace-overlay)))
-            (when (/= s e)
-              (setq delsel--replace-text
-                    (filter-buffer-substring s e))
-              (set-text-properties
-               0 (length delsel--replace-text)
-               nil delsel--replace-text))))
-        (delete-overlay delsel--replace-overlay))
-      (if delsel--replace-text
             (while (and (> count 0)
-                      delsel--replace-text
+                        string
                         (search-forward old-text nil t))
-            (replace-match delsel--replace-text nil t)
+              (replace-match string nil t)
               (setq count (1- count)))
-        (message "Cannot locate replacement text")))))
+          (message "Cannot locate replacement text"))))))
 
 (defun delete-selection-helper (type)
   "Delete selection according to TYPE:

lisp/emacs-lisp/cursor-sensor.el

@@ -155,10 +155,11 @@ By convention, this is a list of symbols where each symbol stands for the
         (let* ((point (window-point window))
                ;; It's often desirable to make the
                ;; cursor-sensor-functions property non-sticky on both
-               ;; ends, so we can't use `get-pos-property' because it
+               ;; ends, so we can't use just `get-pos-property' because it
                ;; might never see it.
                ;; FIXME: Combine properties from covering overlays?
-               (new (or (get-char-property point 'cursor-sensor-functions)
+               (new (or (get-pos-property point 'cursor-sensor-functions)
+                        (get-char-property point 'cursor-sensor-functions)
                         (unless (<= (point-min) point)
                           (get-char-property (1- point)
                                              'cursor-sensor-functions))))
@@ -181,10 +182,10 @@ By convention, this is a list of symbols where each symbol stands for the
                       "Non-nil if F is missing somewhere between START and END."
                       (let ((pos start)
                             (missing nil))
-                        (while (< pos end)
+                        (while (< (setq pos (next-single-char-property-change
-                          (setq pos (next-single-char-property-change
                                              pos 'cursor-sensor-functions
                                              nil end))
+                                  end)
                           (unless (memq f (get-char-property
                                            pos 'cursor-sensor-functions))
                             (setq missing t)))