Inhibit undo-in-region for mouse-drag-region (bug#37700)

'mouse-drag-region' leaves the region active around the dragged text,
so a straight undo did not revert the entire operation.  To remedy
this, inhibit undo-in-region when the last command was
mouse-drag-region.  (Method suggested by Stefan Monnier.)

* lisp/mouse.el (undo-drag-region): Set the undo-inhibit-region property.
* lisp/simple.el (undo): Inhibit undo-in-region if the last command
had the undo-inhibit-region property set.
* doc/lispref/symbols.texi (Standard Properties):
* doc/lispref/text.texi (Undo): Document undo-inhibit-region.
* etc/NEWS: Announce the property.

doc/lispref/symbols.texi

@@ -590,6 +590,11 @@ ignore a call whose value is unused.  If the property's value is
 calls.  In addition to byte compiler optimizations, this property is
 also used for determining function safety (@pxref{Function Safety}).
 
+@item undo-inhibit-region
+If non-@code{nil}, the named function prevents the @code{undo} operation
+from being restricted to the active region, if @code{undo} is invoked
+immediately after the function.  @xref{Undo}.
+
 @item variable-documentation
 If non-@code{nil}, this specifies the named variable's documentation
 string.  This is set automatically by @code{defvar} and related

doc/lispref/text.texi

@@ -1451,6 +1451,12 @@ continuing to undo.
 This function does not bind @code{undo-in-progress}.
 @end defun
 
+Some commands leave the region active after execution in such a way that
+it interferes with selective undo of that command.  To make @code{undo}
+ignore the active region when invoked immediately after such a command,
+set the property @code{undo-inhibit-region} of the command's function
+symbol to a non-nil value.  @xref{Standard Properties}.
+
 @node Maintaining Undo
 @section Maintaining Undo Lists
 

etc/NEWS

@@ -3102,6 +3102,12 @@ in other packages are now obsolete aliases of 'xor'.
 Setting this on the first character of a help string disables
 conversions via 'substitute-command-keys'.
 
++++
+** 'undo' can be made to ignore the active region for a command
+by setting 'undo-inhibit-region' symbol property of that command to
+non-nil.  This is used by 'mouse-drag-region' to make the effect
+easier to undo immediately afterwards.
+
 
 * Changes in Emacs 27.1 on Non-Free Operating Systems
 

lisp/mouse.el

@@ -1104,6 +1104,12 @@ is dragged over to."
     (run-hooks 'mouse-leave-buffer-hook)
     (mouse-drag-track start-event)))
 
+;; Inhibit the region-confinement when undoing mouse-drag-region
+;; immediately after the command.  Otherwise, the selection left
+;; active around the dragged text would prevent an undo of the whole
+;; operation.
+(put 'mouse-drag-region 'undo-inhibit-region t)
+
 (defun mouse-posn-property (pos property)
   "Look for a property at click position.
 POS may be either a buffer position or a click position like

lisp/simple.el

@@ -2508,6 +2508,10 @@ as an argument limits undo to changes within the current region."
 	 (base-buffer (or (buffer-base-buffer) (current-buffer)))
 	 (recent-save (with-current-buffer base-buffer
 			(recent-auto-save-p)))
+         ;; Allow certain commands to inhibit an immediately following
+         ;; undo-in-region.
+         (inhibit-region (and (symbolp last-command)
+                              (get last-command 'undo-inhibit-region)))
 	 message)
     ;; If we get an error in undo-start,
     ;; the next command should not be a "consecutive undo".
@@ -2525,7 +2529,8 @@ as an argument limits undo to changes within the current region."
 		       ;; it shows nothing else happened in between.
 		       (gethash list undo-equiv-table))))
       (setq undo-in-region
-	    (or (region-active-p) (and arg (not (numberp arg)))))
+	    (and (or (region-active-p) (and arg (not (numberp arg))))
+                 (not inhibit-region)))
       (if undo-in-region
 	  (undo-start (region-beginning) (region-end))
 	(undo-start))