GNU bug report logs - #19188
point adjustemnt moves *into* invisible text

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Jonas Bernoulli <jonas <at> bernoul.li>
Subject: bug#19188: closed (Re: bug#19188: point adjustemnt moves *into*
 invisible text)
Date: Wed, 26 Nov 2014 14:52:03 +0000

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Your bug report

#19188: point adjustemnt moves *into* invisible text

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 19188 <at> debbugs.gnu.org.

-- 
19188: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19188
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Jonas Bernoulli <jonas <at> bernoul.li>
Cc: 19188-done <at> debbugs.gnu.org
Subject: Re: bug#19188: point adjustemnt moves *into* invisible text
Date: Wed, 26 Nov 2014 09:51:12 -0500

tags 19188 notabug
thanks

>    However point is not were the cursor is
>      M-: (point) => 3
> The problem is in the code that is supposed to move point *out* of an
> invisible region, does the opposite when moving backward places point
> on the first character after an invisible region.  It moves to the
> beginning of the preceding invisible region.

That is a common misunderstanding.  The fact that point is equal to
3 means that point is *between* character 2 and character 3.  So it's not
*inside* an invisible text, but is right at the boundary.

The position 5 (i.e. between character 4 and character 5) is at the
other end of the boundary.

The reason why Emacs decided to put point at position 3 rather than
leave it at position 5 is because the boundary at position 3 is "less
invisible" than the boundary at position 5.
You can check it with

  M-: (list (get-pos-property 3 'invisible) (get-pos-property 5 'invisible)) RET

This is because text-properties by default are front-non-sticky and
rear-sticky, so if point is at position 5 and you type a character, that
character will inherit the invisible property, whereas if you're at
position 3 and you type a character this character will not inherit the
invisible property.

If you want point to be at position 5 rather than position 3, then you
need to change the front/rear-stickiness of this invisible
property accordingly.

> When point adjustment is disabled (non-nil disable-point-adjustment or
> global-disable-point-adjustment) then this does not happen.

I assume you know why ;-)

> It also does not happen when moving forward, e.g. starting at "1"
> C-p C-f places the cursor on "5" *and* point is also 5.

C-p C-f doesn't do it for me (it doesn't even reach the invisible part of the
text), and if I change the recipe to C-f C-f it doesn't work either
(point stays at position 3).

But indeed C-n gets me to position 5, which is wrong (and doing M-: (point)
returns 5 but moves me to position 3, so doing it again returns 3 :-( ).
So we do have a bug here.


        Stefan

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From: Jonas Bernoulli <jonas <at> bernoul.li>
To: Bug-Gnu-Emacs <bug-gnu-emacs <at> gnu.org>
Subject: point adjustemnt moves *into* invisible text
Date: Wed, 26 Nov 2014 04:07:04 +0100

When moving backward "point adjustment" which is supposed to move
point out of an invisible region may end up doing the opposite.

This problem exists at least in 24.3 and 24.4.

1. Yank this in an empty buffer in fundamental-mode and evaluate it.

(progn
  (goto-char (point-min))
  (insert "1\n"
	  (propertize "3\n" 'invisible t)
	  "5\n"
	  "7\n")
	  (backward-char 2))

2. The cursor is now on the "7", which also is the 7th character.

   The buffer looks like
   .-----
   |1
   |5
   |7
   |(progn
   |...
   `-----

   The cursor sits on the "7" and

     M-: (point) => 7
   
3. Move to "5" using e.g. C-p or C-b C-b.

   The cursor is now on "5", which also is the 5th character.

   However point is not were the cursor is

     M-: (point) => 3

The problem is in the code that is supposed to move point *out* of an
invisible region, does the opposite when moving backward places point
on the first character after an invisible region.  It moves to the
beginning of the preceding invisible region.

When point adjustment is disabled (non-nil disable-point-adjustment or
global-disable-point-adjustment) then this does not happen.

It also does not happen when moving forward, e.g. starting at "1"
C-p C-f places the cursor on "5" *and* point is also 5.

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