bug#66780: [PATCH] Improve rectangle-mark-mode when transient-mark-mode is off

[Jens Schmidt]

Severity: minor

All issues reported here are about using rectangle-mark-mode (RMM) when
Transient Mark mode (TMM) is off.

Bug#42663 already has mentioned a few issues of this combination, but
IMO did not completely describe or solve them.


So suppose you start in the Emacs source directory tree

  ./src/emacs -Q --eval='(transient-mark-mode -1)' README

When you then press `C-x <SPC>' to enable RMM, the following happens,
which is expected:

1. The mark is NOT activated and, hence, the region-rectangle is NOT
   shown when you move point.  Also, point movement stays regular, in
   that you cannot advance point to after EOL, for example.

Again, this restriction is expected and is described already in commit
06dcd2be5d42 provided by Sean to solve bug#42663.


However, what has not been described yet is that some parts of RMM
actually _are_ active when one enables it with TMM off:

2. The key bindings in `rectangle-mark-mode-map', so that, for example,
   `C-x C-x' does not exchange point and mark, but instead cycles
   through the corners of the rectangle spanned by mark and point (using
   closest accessible buffer positions as an approximation for those
   positions where point regularly cannot be moved to).

   (Short reproducer when starting off the "emacs -Q ..." above:
   M-: (rectangle-mark-mode -1) RET
   M-<   C-x <SPC>   C-4 C-n   C-e   C-x C-x   C-x C-x   C-x C-x)

3. Advices on `region-extract-function' and `region-insert-function', so
   that C-w and C-y still operate on the current rectangle, and not the
   regular region.

   (Short reproducer when starting off the "emacs -Q ..." above:
   M-: (rectangle-mark-mode -1) RET
   M-<   C-x <SPC>   C-4 C-n   C-4 C-f   C-w)


I found 2. and 3. surprising because I would have expected that RMM is
completely off.  Moreover, RMM does not provide a minor mode lighter so
you cannot easily tell whether it is on or off.

I have attempted to improve the situation by providing documentation
(fix A) in a patch on emacs-29, and some minor code fixes (B and C) in a
patch on emacs-master.  That latter patch still has a placeholder
"bug#XXXXX" which needs to be updated once I have a bug number.


* Fix A: Improve Documentation

Here is the text that Sean has already added to emacs/killing.tex in
commit 06dcd2be5d42:

+The region-rectangle works only when the mark is active.  In
+particular, when Transient Mark mode is off (@pxref{Disabled Transient
+Mark}), in addition to typing @kbd{C-x @key{SPC}} you will need to
+activate the mark.

I first tried to extend on that, but then decided to completely redo the
material, like this:

+  rectangle-mark-mode behaves in a slightly different way when
+Transient Mark mode is off (@pxref{Disabled Transient Mark}).  In this
+case, when you enable rectangle-mark-mode, the region-rectangle is not
+automatically enabled.  Accordingly, cursor movement with @kbd{C-f},
+@kbd{C-n} etc.@: is confined to the regularly accessible buffer
+positions.  However, killing and yanking still operate on the
+rectangle spanned by point and mark.  Also @kbd{C-x C-x} still cycles
+through the corners of that rectangle, but only as far as these are at
+buffer positions that are regularly accessible.
+
+  As mentioned above, rectangle-mark-mode persists as long the region
+is active: If the region gets deactivated, rectangle-mark-mode gets
+deactivated as well.  But with disabled Transient mark mode there is
+usually no active region that would get deactivated, and so you have
+to explicitly switch off rectangle-mark-mode when you no longer want
+to use it.
+
+  To experience all benefits of rectangle-mark-mode and the
+region-rectangle when Transient Mark mode is off, you can temporarily
+activate Transient Mark mode after enabling rectangle-mark-mode, for
+example, with @kbd{C-@key{SPC} C-@key{SPC}}.  @xref{Disabled Transient
+Mark}.


I also replaced the somewhat unhelpful reference to parent node
"@xref{Killing}" further up by references to sibling nodes:

 so in a rectangular fashion, and killing and yanking operate on the
-rectangle.  @xref{Killing}.  The mode persists only as long as the
-region is active.
+rectangle.  @xref{Deletion and Killing}, @ref{Yanking}.  The mode
+persists only as long as the region is active.


* Fix B: Use a Minor Mode Lighter

With TMM off and RMM on, the most important indicator of RMM (the
region-rectangle) is not visible.  And since RMM's minor mode lighter is
nil, one has no visible feedback of it still being active.  That can be
surprising since some of its features are still active as described
above.

I tried to improve that by using a minor mode lighter that goes on only
if TMM is off, like this:

+(defvar rectangle-mark-mode-lighter nil
+  "Lighter displayed for `rectangle-mark-mode'.")
+
 ;;;###autoload
 (define-minor-mode rectangle-mark-mode
   "Toggle the region as rectangular.

 Activates the region if it's inactive and Transient Mark mode is
 on.  Only lasts until the region is next deactivated."
-  :lighter nil
+  :lighter rectangle-mark-mode-lighter
   (rectangle--reset-crutches)
   (when rectangle-mark-mode
+    ;; Make us more visible when Transient Mark mode is off and there
+    ;; is no rectangle (bug#XXXXX).
+    (setq rectangle-mark-mode-lighter
+          (and (not transient-mark-mode) " Rect"))


* Fix C: Mark RMM Movement Commands as Shift-Selectable

Finally, to make RMM better usable with shift-select-mode, I added the
necessary interactive specifiers to the RMM-specific movement commands

  rectangle-right-char
  rectangle-left-char
  rectangle-forward-char
  rectangle-backward-char
  rectangle-next-line
  rectangle-previous-line


Please review.

Thanks.

0001-29-Improve-rectangle-mark-mode-when-transient-mark-mode.patch
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0001-30-Improve-rectangle-mark-mode-when-transient-mark-mode.patch
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