bug#35434: [<8; 36; 7m26.2; terminal emacs: continuation line indicator missing when word wrapping is enabled

bug#35434: [<8; 36; 7m26.2; terminal emacs: continuation line indicator missing when word wrapping is enabled

[Luis Gerhorst]

1. Start Emacs using 'emacs -Q --no-window-system'.

2. Reduce the window width until the lines in the scratch buffer wrap
around. Notice that Emacs shows a '\' at the end of each wrapped line.

3. Type M-x toggle-word-wrap. Emacs no longer shows a '\' to indicate
continuation lines. There is no way to tell whether the line contains a
newline, or whether it has been wrapped.

Notice that if you do the same in graphical Emacs (i.e. start with
'emacs -Q'), the fringes do show an indicator for continuation lines,
even with word wrapping enabled.

This is no critical bug, however, the behavior is inconsistent and not
user friendly. The following post shows, that it has also confused other
users who noticed the inconsistency:

https://stackoverflow.com/questions/12989072/showing-continuation-lines-in-emacs-in-a-text-terminal

If you think, this should not be a bug report and can point me to a
better place to discuss the issue, I will be happy to do so.

Best regards,
Luis


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bug#35434: terminal emacs: continuation line indicator missing when word wrapping is enabled

[Luis Gerhorst]

Pleasy excuse me for the artifacts in the subject, it should have been 'terminal emacs: continuation line indicator missing when word wrapping is enabled'.

bug#35434: [<8; 36; 7m26.2; terminal emacs: continuation line indicator missing when word wrapping is enabled

[Eli Zaretskii]

> From: Luis Gerhorst <privat@luisgerhorst.de>
> Date: Fri, 26 Apr 2019 10:07:58 +0200
> 
> 1. Start Emacs using 'emacs -Q --no-window-system'.
> 
> 2. Reduce the window width until the lines in the scratch buffer wrap
> around. Notice that Emacs shows a '\' at the end of each wrapped line.
> 
> 3. Type M-x toggle-word-wrap. Emacs no longer shows a '\' to indicate
> continuation lines. There is no way to tell whether the line contains a
> newline, or whether it has been wrapped.
> 
> Notice that if you do the same in graphical Emacs (i.e. start with
> 'emacs -Q'), the fringes do show an indicator for continuation lines,
> even with word wrapping enabled.

The canonical way of using this feature is with "M-x visual-line-mode",
not with toggle-word-wrap.  visual-line-mode takes care of removing
the fringe indicators as well, so the appearance on GUI frames and TTY
frames is the same: without continuation indicators.

toggle-word-wrap does part of the job of visual-line-mode, and frankly
I don't understand why the former even exists.

Making continuation glyphs appear in TTY frames shouldn't be hard, but
I don't know how important is that.

bug#35434: terminal emacs: continuation line indicator missing when word wrapping is enabled

[Luis Gerhorst]

Eli Zaretskii writes:

> Making continuation glyphs appear in TTY frames shouldn't be 
> hard, but
> I don't know how important is that.

Would you accept a patch if I manage to create one?

bug#35434: terminal emacs: continuation line indicator missing when word wrapping is enabled

[Eli Zaretskii]

> From: Luis Gerhorst <privat@luisgerhorst.de>
> Cc: 35434@debbugs.gnu.org
> Date: Fri, 26 Apr 2019 13:29:49 +0200
> 
> Eli Zaretskii writes:
> 
> > Making continuation glyphs appear in TTY frames shouldn't be 
> > hard, but
> > I don't know how important is that.
> 
> Would you accept a patch if I manage to create one?

I will definitely review it.  If it's clean, and if its smaller than
about 15 lines, we can accept it; longer patches will need you to
assign copyright to the FSF.

Would you perhaps like to describe the idea behind the patch you would
like to submit before actually coding it?  This could avoid the need
to rework it more than necessary.

Thanks.

bug#35434: terminal emacs: continuation line indicator missing when word wrapping is enabled

[Luis Gerhorst]

Eli Zaretskii writes:

>> From: Luis Gerhorst <privat@luisgerhorst.de>
>> Cc: 35434@debbugs.gnu.org
>> Date: Fri, 26 Apr 2019 13:29:49 +0200
>>
>> Eli Zaretskii writes:
>>
>> > Making continuation glyphs appear in TTY frames shouldn't be
>> > hard, but
>> > I don't know how important is that.
>>
>> Would you accept a patch if I manage to create one?
>
> I will definitely review it.  If it's clean, and if its smaller 
> than
> about 15 lines, we can accept it; longer patches will need you 
> to
> assign copyright to the FSF.
>
> Would you perhaps like to describe the idea behind the patch you 
> would
> like to submit before actually coding it?  This could avoid the 
> need
> to rework it more than necessary.

Code-wise I have no idea yet ;)

My initial approach would be to have a look at the patches that
introduced word wrapping in the first place. If I'm lucky they 
modified
the GUI code to still show the fringe indicators, but simply 
missed the
TUI code.

However, I've not read any Emacs C code ever. I'll look into it 
and come
back when I unserstand the problem.

bug#35434: terminal emacs: continuation line indicator missing when word wrapping is enabled

[Eli Zaretskii]

> From: Luis Gerhorst <privat@luisgerhorst.de>
> Cc: 35434@debbugs.gnu.org
> Date: Fri, 26 Apr 2019 13:49:08 +0200
> 
> > Would you perhaps like to describe the idea behind the patch you
> > would like to submit before actually coding it?  This could avoid
> > the need to rework it more than necessary.
> 
> Code-wise I have no idea yet ;)
> 
> My initial approach would be to have a look at the patches that
> introduced word wrapping in the first place. If I'm lucky they
> modified the GUI code to still show the fringe indicators, but
> simply missed the TUI code.
> 
> However, I've not read any Emacs C code ever. I'll look into it and
> come back when I unserstand the problem.

Look at the function produce_special_glyphs in xdisp.c.

I think showing the continuation glyphs on TTY frames in word-wrap
mode should be optional, controlled by a user variable, since the
current behavior is unchanged since Emacs 23.

Thanks.