GNU bug report logs - #45319
28.0.50; Redisplay: `forward-char' moves backwards into invisible text

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Subject: bug#45319: closed (Re: bug#45319: 28.0.50; Redisplay:
 `forward-char' moves backwards into invisible text)
Date: Sun, 20 Dec 2020 15:39:02 +0000

[Message part 1 (text/plain, inline)]

Your bug report

#45319: 28.0.50; Redisplay: `forward-char' moves backwards 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 45319 <at> debbugs.gnu.org.

-- 
45319: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=45319
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: jonas <at> bernoul.li, 45319-done <at> debbugs.gnu.org
Subject: Re: bug#45319: 28.0.50; Redisplay: `forward-char' moves backwards
 into invisible text
Date: Sun, 20 Dec 2020 17:38:26 +0200

> From: Michael Heerdegen <michael_heerdegen <at> web.de>
> Cc: 45319 <at> debbugs.gnu.org,  jonas <at> bernoul.li
> Date: Sun, 20 Dec 2020 09:01:52 +0100
> 
> I see now that that problem has been discussed in Bug#23079.  If you
> think that everything is alright with my test case you might close this
> bug and probably merge with that one.

I closed this one.  Merging them is too much of a hassle with
debbugs's unhelpful defaults for archived bugs.

The only other thing I could say, which may or may not be helpful for
your use case, is that Lisp programs shouldn't assume they will always
find point outside of invisible portion of the text, and instead
actively look for the first visible position after point.

[Message part 3 (message/rfc822, inline)]

From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: bug-gnu-emacs <at> gnu.org
Cc: Jonas Bernoulli <jonas <at> bernoul.li>
Subject: 28.0.50; Redisplay: `forward-char' moves backwards into invisible text
Date: Sat, 19 Dec 2020 07:10:53 +0100

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Hello,

I have found a strange problem I encountered in the Magit Log buffer,
but it is reproducible in any buffer in emacs -Q.  CC'ing Jonas
Bernoulli, maintainer of Magit, for information.

I insert something like this into e.g. *scratch* (looks a bit like a
Magit log buffer):

[test.txt (text/plain, inline)]

11111
  a
  b
  c
22222
  x
  y
  z
33333
  f
  g
  h
44444
  i
  j
  k

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Lets do this:

  (add-hook 'post-command-hook (lambda () (message "%s" (point))))

to see the value of point when we move around in the buffer.

I want to hide lines that start with whitespace, so I do this:

  (defvar my-magit-log-show-only-hash-lines-mode-keywords
    `((,(rx (and bol space (* any) eol))
       (0 (let ((beg (match-beginning 0))
                (end (1+ (match-end 0))))
            (add-text-properties beg end
                                 (list 'invisible 'my-magit-log-show-only-hash-lines))
            nil)))))

  (define-minor-mode my-magit-log-show-only-hash-lines-mode
    "..." nil nil nil
    (if (not my-magit-log-show-only-hash-lines-mode)
        (remove-from-invisibility-spec 'my-magit-log-show-only-hash-lines)
      (font-lock-add-keywords nil my-magit-log-show-only-hash-lines-mode-keywords)
      (add-to-invisibility-spec 'my-magit-log-show-only-hash-lines)))

Now enable that mode in the buffer and M-x font-lock-fontify-buffer.

When I now move around in the buffer with the arrow keys, it often
happens that when point is at the beginning of a visible line, and I hit
<right>, the echo area shows that point actually has moved backwards into
the invisible area somewhere before that line, but the cursor is placed
at the expected position.  Further movement behaves as expected from the
drawn cursor position, just the value of `point' sometimes is
inconsistent, and that can cause trouble (like incorrect section
highlighting in the Magit Log buffer).


TIA,

Michael.

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