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

From: Michael Heerdegen <[email-masked]>
Date: Sat, 19 Dec 2020 07:10:53 +0100

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):

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

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.