bug#57433: Emacs no longer moves point into visible port of the buffer

[Eli Zaretskii <eliz@gnu.org>]

> From: Paul Pogonyshev <pogonyshev@gmail.com>
> Date: Sat, 27 Aug 2022 18:06:59 +0200
> Cc: Gerd Möllmann <gerd.moellmann@gmail.com>, 
> 	57433-done@debbugs.gnu.org
> 
> Setup: Magit buffer with staged files, at least two. It looks like this:
> 
> Staged changes (N)
> modified    file1
> [invisible diff of file1]
> modified    file2
> [invisible diff of file2]
> 
> so essentially I, as a user, only see this in UI:
> 
> Staged changes (N)
> modified    file1
> modified    file2
> 
> Now, I put the point at the beginning of the third line ([^] is the point):
> 
> Staged changes (N)
> modified    file1
> [^]modified    file2
> 
> I press C-b, expecting that the point is moved to the end of the
> previous line that I see. But it is not moved there, this is the result:
> 
> Staged changes (N)
> modified    file1
> [^]modified    file2
> 
> In reality, the point is in the invisible (to me, as the user) diff for file1.
> If I press C-b again, only now point is moved like I expected it to be
> moved the last time:
> 
> Staged changes (N)
> modified    file1[^]
> modified    file2

I cannot reproduce this, if I try to simulate it by having those 4
lines with two lines invisible.  So I think there's something else at
work here.  If you can show a simple recipe that doesn't involve
Magit, I can look into what happens there.  (And it probably warrants
a separate bug report.)

What I did was to have this text:

  modified    file1
  [invisible diff of file1]
  modified    file2
  [invisible diff of file2]

(without the indentation) in a buffer, and then I manually put the
invisible text property on the 2nd and the 4th lines.  Then I moved
the cursor with C-b from the beginning of "modified file2", and the
cursor was displayed at the end of the previous visible line, the one
which says "modified file1".

> So, the presence of invisible text between lines 2 and 3 changes the
> way C-b behaves and breaks my (as a user of Emacs) expectations,
> disrupting established editing flow. I'd say that the invisible text
> should count as implementation details and shouldn't affect user-
> visible results at all.
> 
> What do you think?

It works for me the way you expect it to work, so I'm not yet sure
what's going on in the Magit buffer.