bug#6032: untabify causes point to move

bug#6032: untabify causes point to move

[Ethan]

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`untabify' sometimes causes the cursor to jump. If you have a buffer
like this, with (-!-) representing point:

^I(-!-)^ISome text some text some text

And (for example) have a before-save-hook that calls untabify on the
whole buffer, the cursor will jump to the beginning of line. This is a
bit surprising.



In GNU Emacs 23.1.1 (i486-pc-linux-gnu, GTK+ Version 2.18.3)
 of 2010-03-26 on palmer, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.10604000
configured using `configure  '--build=i486-linux-gnu'
'--host=i486-linux-gnu' '--prefix=/usr' '--sharedstatedir=/var/lib'
'--libexecdir=/usr/lib' '--localstatedir=/var/lib'
'--infodir=/usr/share/info' '--mandir=/usr/share/man' '--with-pop=yes'
'--enable-locallisppath=/etc/emacs23:/etc/emacs:/usr/local/share/emacs/23.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.1/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.1/leim'
'--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars'
'build_alias=i486-linux-gnu' 'host_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -g
-O2' 'LDFLAGS=-g' 'CPPFLAGS=''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  yas/global-mode: t
  yas/minor-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
M-x r e p o r t - e <tab> <return>

Recent messages:
Ido mode enabled
Failed to load jxp mode: (file-error Cannot open load file jxp-mode)
/usr/bin/mail is not an executable.  Setting mail-interactive to t.
For information about GNU Emacs and the GNU system, type C-h C-a.
Saving all Org-mode buffers...
(No files need saving)
Saving all Org-mode buffers... done
Saving all Org-mode buffers...
(No files need saving)
Saving all Org-mode buffers... done

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bug#6032: untabify causes point to move

[Juanma Barranquero]

On Sun, Apr 25, 2010 at 02:01, Ethan <ethan.glasser.camp@gmail.com> wrote:
> `untabify' sometimes causes the cursor to jump. If you have a buffer
> like this, with (-!-) representing point:
>
> ^I(-!-)^ISome text some text some text
>
> And (for example) have a before-save-hook that calls untabify on the
> whole buffer, the cursor will jump to the beginning of line. This is a
> bit surprising.

It can also happen in interactive use. In your example, if you select
the first tab, with point just at the second one, and do M-x untabify,
the cursor jumps to the beginning of the line.

All in all, it is a bit surprising. If you put the cursor at the first
t (in text), for example, the cursor does not jump. Why not?
`save-excursion' saves the point, but the point was 8 and it is now
22...

The problem can be trivially fixed by wrapping the tabify code in

  (let ((c (current-column)))
     ;;; tabify
     (move-to-column c))

as in the following patch, but perhaps there are cleaner ways.

(You can do that in an around advice, as a workaround.)

    Juanma


=== modified file 'lisp/tabify.el'
--- lisp/tabify.el	2010-01-13 08:35:10 +0000
+++ lisp/tabify.el	2010-04-25 22:53:06 +0000
@@ -34,19 +34,21 @@
 START and END, rather than by the position of point and mark.
 The variable `tab-width' controls the spacing of tab stops."
   (interactive "r")
-  (save-excursion
-    (save-restriction
-      (narrow-to-region (point-min) end)
-      (goto-char start)
-      (while (search-forward "\t" nil t)	; faster than re-search
-	(forward-char -1)
-	(let ((tab-beg (point))
-	      (indent-tabs-mode nil)
-	      column)
-	  (skip-chars-forward "\t")
-	  (setq column (current-column))
-	  (delete-region tab-beg (point))
-	  (indent-to column))))))
+  (let ((c (current-column)))
+    (save-excursion
+      (save-restriction
+        (narrow-to-region (point-min) end)
+        (goto-char start)
+        (while (search-forward "\t" nil t)      ; faster than re-search
+          (forward-char -1)
+          (let ((tab-beg (point))
+                (indent-tabs-mode nil)
+                column)
+            (skip-chars-forward "\t")
+            (setq column (current-column))
+            (delete-region tab-beg (point))
+            (indent-to column)))))
+    (move-to-column c)))

 (defvar tabify-regexp " [ \t]+"
   "Regexp matching whitespace that tabify should consider.

bug#6032: untabify causes point to move

[Lars Magne Ingebrigtsen]

Juanma Barranquero <lekktu@gmail.com> writes:

> The problem can be trivially fixed by wrapping the tabify code in
>
>   (let ((c (current-column)))
>      ;;; tabify
>      (move-to-column c))
>
> as in the following patch, but perhaps there are cleaner ways.

I think that's the easiest way to preserve point.  I've now applied the
patch to Emacs 24.

-- 
(domestic pets only, the antidote for overdose, milk.)
  bloggy blog http://lars.ingebrigtsen.no/