delete-selection-mode: Add user option to delete temporary regions only

* lisp/delsel.el (delete-selection-temporary-region): Add new
user option.
(delete-selection-pre-hook): Respect it.
* doc/emacs/mark.texi (Using Region): Document the new user option.
* etc/NEWS: Announce the new user option. (bug#55692)

doc/emacs/mark.texi

@@ -291,12 +291,23 @@ instead signal an error if the mark is inactive.
 @cindex Delete Selection mode
 @cindex mode, Delete Selection
 @findex delete-selection-mode
+@vindex delete-selection-temporary-region
   By default, text insertion occurs normally even if the mark is
 active---for example, typing @kbd{a} inserts the character @samp{a},
 then deactivates the mark.  Delete Selection mode, a minor mode,
 modifies this behavior: if you enable that mode, then inserting text
 while the mark is active causes the text in the region to be deleted
-first.  To toggle Delete Selection mode on or off, type @kbd{M-x
+first.  However, you can tune this behavior by customizing the
+@code{delete-selection-temporary-region} option.  Its default value is
+@code{nil}, but you can set it to @code{t}, in which case only
+temporarily-active regions will be replaced: those which are set by
+dragging the mouse (@pxref{Setting Mark}) or by shift-selection
+(@pxref{Shift Selection}), as well as by @kbd{C-u C-x C-x} when
+Transient Mark Mode is disabled.  You can further tune the behavior by
+setting @code{delete-selection-temporary-region} to @code{selection}:
+then temporary regions by @kbd{C-u C-x C-x} won't be replaced, only
+the ones activated by dragging the mouse or shift-selection.  To
+toggle Delete Selection mode on or off, type @kbd{M-x
 delete-selection-mode}.
 
 @node Mark Ring

etc/NEWS

@@ -271,6 +271,11 @@ startup.  Previously, these functions ignored
 
 * Changes in Emacs 29.1
 
++++
+** New user option 'delete-selection-temporary-region'.
+When non-nil, 'delete-selection-mode' will only delete the temporary
+regions (usually set by mouse-dragging or shift-selection).
+
 +++
 ** New user option 'switch-to-prev-buffer-skip-regexp'.
 This should be a regexp or a list of regexps; buffers whose names

lisp/delsel.el

@@ -64,6 +64,19 @@
   "If non-nil, deleted region text is stored in this register.
 Value must be the register (key) to use.")
 
+(defcustom delete-selection-temporary-region nil
+  "Whether to delete only temporary regions.
+When non-nil, typed text replaces only the regions set by
+mouse-dragging, shift-selection, and \"\\[universal-argument] \\[exchange-point-and-mark]\" when
+`transient-mark-mode' is turned off.  If the value is the symbol
+`selection', then replace only the regions set by mouse-dragging
+and shift-selection."
+  :version "29.1"
+  :group 'editing-basics
+  :type '(choice (const :tag "Replace all regions" nil)
+                 (const :tag "Replace region from mouse, shift-selection, and \"C-u C-x C-x\"" t)
+                 (const :tag "Replace region from mouse and shift-selection" selection)))
+
 ;;;###autoload
 (defalias 'pending-delete-mode 'delete-selection-mode)
 
@@ -251,7 +264,13 @@ property on their symbol; commands which insert text but don't
 have this property won't delete the selection.
 See `delete-selection-helper'."
   (when (and delete-selection-mode (use-region-p)
-	     (not buffer-read-only))
+	     (not buffer-read-only)
+             (or (null delete-selection-temporary-region)
+                 (and delete-selection-temporary-region
+                      (consp transient-mark-mode)
+                      (eq (car transient-mark-mode) 'only))
+                 (and (not (eq delete-selection-temporary-region 'selection))
+                      (eq transient-mark-mode 'lambda))))
     (delete-selection-helper (and (symbolp this-command)
                                   (get this-command 'delete-selection)))))