visual-line-mode produces erratic behaviour with text properties

visual-line-mode produces erratic behaviour with text properties

[Paul Rankin]

Enabling visual-line-mode and adding line-prefix and wrap-prefix properties to a long paragraph of text produces erratic navigational behaviour. Can anyone else reproduce?

To reproduce:

1. $ emacs -Q

2. insert a long paragraph of text
M-: (url-insert-file-contents "https://gist.githubusercontent.com/rnkn/d5987c3495901e4950782f6648846e4e/raw/c22074e8f0940f6ec9c8b729f04567dc50664823/loren-ipsum.txt")

3. turn on visual line mode
M-: (visual-line-mode 1)

4. add text properties
M-: (add-text-properties (point-min) (point-max) '(line-prefix (space :align-to 10) wrap-prefix (space :align-to 10)))

5. C-n past window length and up and around.

Expected results:

Navigation should continue by visual line.

Actual results:

Navigational behaviour becomes erratic. Point will no long move with visual lines, instead jumping to different columns. This is more pronounced with a window 80 columns or less.
 
Configuration:

GNU Emacs 24.5.1 (x86_64-apple-darwin15.5.0, NS apple-appkit-1404.47) of 2016-06-26

OS X 10.11.4 (15E65)

MacBook Pro (Retina, 15-inch, Mid 2014)
2.2 GHz Intel Core i7
16 GB 1600 MHz DDR3
Intel Iris Pro 1536 MB

-- 
Paul W. Rankin
www.paulwrankin.com

Re: visual-line-mode produces erratic behaviour with text properties

[Eli Zaretskii]

> From: Paul Rankin <hello@paulwrankin.com>
> Date: Fri, 01 Jul 2016 15:30:25 +1000
> 
> 1. $ emacs -Q
> 
> 2. insert a long paragraph of text
> M-: (url-insert-file-contents "https://gist.githubusercontent.com/rnkn/d5987c3495901e4950782f6648846e4e/raw/c22074e8f0940f6ec9c8b729f04567dc50664823/loren-ipsum.txt")
> 
> 3. turn on visual line mode
> M-: (visual-line-mode 1)
> 
> 4. add text properties
> M-: (add-text-properties (point-min) (point-max) '(line-prefix (space :align-to 10) wrap-prefix (space :align-to 10)))
> 
> 5. C-n past window length and up and around.
> 
> Expected results:
> 
> Navigation should continue by visual line.
> 
> Actual results:
> 
> Navigational behaviour becomes erratic. Point will no long move with visual lines, instead jumping to different columns. This is more pronounced with a window 80 columns or less.

Not quite accurate: the erratic behavior begins only when point
reaches the end of the first window-full.

Please report this as a bug, using "M-x report-emacs-bug RET".
Super-long lines such as this one slow down the display engine, and
there are various redisplay optimizations to try to countermand that,
so perhaps this is one of them.

In general, my recommendation is to stay away of such long lines in
Emacs.

Re: visual-line-mode produces erratic behaviour with text properties

[Paul Rankin]

On Fri, 1 Jul 2016, at 05:55 PM, Eli Zaretskii wrote:
> Super-long lines such as this one slow down the display engine, and
> there are various redisplay optimizations to try to countermand that,
> so perhaps this is one of them.

Do you mean Emacs has redisplay optimisations that may be causing the erratic navigation?

Or, are there redisplay optimisations I can implement myself in an affected major mode to work around this?

> In general, my recommendation is to stay away of such long lines in
> Emacs.

I can't control this for users of the major mode. It's an edge case, but still.

Thanks

Re: visual-line-mode produces erratic behaviour with text properties

[Eli Zaretskii]

> From: Paul Rankin <hello@paulwrankin.com>
> Date: Fri, 01 Jul 2016 18:29:40 +1000
> 
> On Fri, 1 Jul 2016, at 05:55 PM, Eli Zaretskii wrote:
> > Super-long lines such as this one slow down the display engine, and
> > there are various redisplay optimizations to try to countermand that,
> > so perhaps this is one of them.
> 
> Do you mean Emacs has redisplay optimisations that may be causing the erratic navigation?

Yes, at least in principle.

> Or, are there redisplay optimisations I can implement myself in an affected major mode to work around this?

That's not the way to deal with these problems.  If some redisplay
optimization causes incorrect display in some use case, it should be
disabled in that use case.  IOW, it's a bug that needs to be fixed,
not worked around in application code.

> > In general, my recommendation is to stay away of such long lines in
> > Emacs.
> 
> I can't control this for users of the major mode. It's an edge case, but still.

That recommendation was mainly for end-users, not for developers.  Of
course, if you can come up with an implementation that avoids long
lines, you as developer should prefer it, but I understand this is not
such a case.