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

[Eli Zaretskii <eliz@gnu.org>]

> 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.