bug#64988: 30.0.50; move-to-column can move across lines if there is a text with display property

[Herman, Geza]

I think, in my case, the move-to-column should completely ignore the 
display property. Is there a function which does that?

I noticed this problem because how indent-bars interacts with 
column-enforce-mode (https://github.com/jordonbiondo/column-enforce-mode).

column-enforce-mode uses move-to-column to mark long lines. And because 
the display property with "\n" basically joins neighboring lines, 
column-enforce-mode marks lines incorrectly.

I think it should only consider what's in the buffer, and ignore all 
rendering related properties. Is there a way to accomplish this?

Regarding how to fix this problem: wouldn't it make sense to stop 
calculating the width at the first "\n" in the displayed string? At 
least, it would fix this problem, as far as I understand. Not sure if it 
breaks anything. But supposedly nothing should depend on the exact 
behavior of column calculations, if the displayed string contains 
characters after the first "\n" (because it's kind of undefined what the 
width of a multi-line string is)

On 8/1/23 14:25, Eli Zaretskii wrote:
> tags 64988 notabug wontfix
> thanks
>
> > Date: Tue, 1 Aug 2023 12:53:57 +0200
> > From: Herman@debbugs.gnu.org, Géza <geza.herman@gmail.com>
> >
> > If there is a text with display property in a buffer, then
> > move-to-column can move across lines.
> >
> > Repro:
> > * emacs -Q
> > * in the scratch buffer, move the cursor to the top, and put an empty
> > line at the beginning of the buffer
> > * execute 'M-: (put-text-property 1 2 'display "line\n")' (note: it's
> > likely that the "\n" causes the problem)
> > * notice that the empty line becomes "line"
> > * while the cursor still on the first line, execute 'M-: (move-to-column
> > 20)'
> >
> > The last command will move the cursor to the next line at column 16,
> > instead of staying at the first line.
> >
> > Note: I noticed this problem while using this package:
> > https://github.com/jdtsmith/indent-bars.
> >
> > The problem doesn't happen with emacs 28, this is the commit that
> > introduced the issue:
> >
> > 4243747b1b8c3b7e3463822804b32e83febe2878 Fix 'current-column' in the
> > presence of display strings
> This is a known issue, but it is not a bug, at least not one we know
> how to "fix".  In general, move-to-column cannot work correctly when
> display strings with embedded newlines are involved, because Emacs
> cannot place the cursor at the newline in the middle of such a display
> string.
>
> The commit to which you point fixed current-column to correctly report
> the column where such display strings are involved, at least in the
> important cases.  That move-to-column changed behavior in those cases
> is unfortunate, but I couldn't find any sensible way of dealing with
> these cases, because the correct result -- setting the cursor after
> the newline embedded in the display string, because column 20 doesn't
> exist on that line -- is impossible.  (The result that you expected,
> which was what Emacs 28 did, i.e. for the cursor to stay at buffer
> position 1, is also incorrect, since that position is column 1, not
> column 20 and not the last column visible on that line.)
>
> So I'm sorry, but unless someone comes with an idea for how to handle
> these situations in a sensible way (and I thought long and hard about
> it, but couldn't find such ideas), this will remain a limitation of
> move-to-column, and one of the complications introduced by display
> strings with embedded newlines in general.  The affected packages will
> have to adapt to this change in some ways.  Since the original code
> also produced incorrect results, just different incorrect results, I
> don't see this as a serious problem, just as a bug that wasn't fixed,
> but changed its buggy behavior.