Re: `vertical-motion', `goto-line' set point to invisible text

[Dmitry Kurochkin <dmitry.kurochkin@gmail.com>]

Hi Eli.

On Sun, 03 Jul 2011 02:55:31 -0400, Eli Zaretskii <eliz@gnu.org> wrote:
> > From: Dmitry Kurochkin <dmitry.kurochkin@gmail.com>
> > Date: Sun, 03 Jul 2011 06:59:49 +0400
> > 
> > While working on notmuch [1] emacs client, I stumbled upon an unexpected
> > behavior: `beginning-of-visual-line' places point to invisible text in
> > the beginning to the line.  I.e. you have:
> > 
> >   line1
> >   line2    <--- this line is not visible
> >   line3    <--- point is on this line
> > 
> > in this case, `beginning-of-visual-line' will set the position to start
> > of line2 (which is invisible), not line3.  It differs from what
> > `move-beginning-of-line' and even does not match
> > `line-beginning-position'.  `beginning-of-visual-line' uses
> > `vertical-motion' to do the job.  `goto-line' has a similar behavior.  I
> > believe there are more functions like this.
> 
> See the node "Invisible Text" in the ELisp manual.  Some functions are
> explicitly programmed to special behavior in the vicinity of invisible
> text, others aren't.
> 

Thanks for the hint.

> > This does not look right to me.  I expect these functions never set
> > point inside invisible text and there should be some general way to
> > protect from this.
> 
> Are you sure you don't mix invisible with intangible?

Yes, I know invisible is not the same as intangible.

>  Invisible means
> just that: not shown on the screen.  It doesn't mean point cannot
> enter the invisible portion.  Emacs does try to move point out of
> invisible range of text, but it does so in its command loop, _after_
> the cursor motion functions and the display engine did their job.

I guess that explains why in notmuch (which has multiple lines hidden,
i.e. email body) I hit C-a, then M-: "(point)" I get X, then if I do M-:
"(point)" again, I get Y.  Is there a way to ask Emacs to sync point to
visible position, so that I do not have to do it by hand?

>  So
> your test program just shows that cursor motion has no problem getting
> into the invisible region, which I think is not a bug.
> 
> > But perhaps I am wrong and we have to manually skip
> > all invisible text forward after any point move?
> 
> Yes.
> 

Thanks for your answers.

> There is something strange in how we behave in this example, though.
> To see it, modify your test program to make each line's 1st character
> different from other lines.  Then, after the program ends, C-a on the
> 3rd line and type "C-x =": Emacs says the character at point is the
> first character of the invisible line 2, which seems wrong, as the
> display shows line 3.  In fact, you cannot place point on the 1st
> character of line 3: C-b from the 2nd character of line 3 gets you to
> the 1st character of line 2!  I think this happens because the range
> of invisible characters includes the newline of line 2.

Indeed, I did not notice that when testing in notmuch (I guess because
there are many lines hidden).

`backward-char' steps 1 position as expected.  I.e. M-: (progn
(backward-char) (point)) works fine.  But after that M-: (point) returns
another position which corresponds to the beginning of the hidden line.
I guess this has to do command loop I know nothing about.  IMO Emacs
should not touch the point in this case, because it is already visible.
Sounds like a bug to me.

Regards,
  Dmitry