bug#23879: visual-line-mode produces erratic behaviour with text properties

[Eli Zaretskii]

> From: Paul Rankin <hello@paulwrankin.com>
> Date: Fri, 01 Jul 2016 18:33:48 +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 height.
> 
> 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.

Initial investigation indicates that this is a very old problem: I see
it in Emacs 23.1, which is when visual-line-mode was introduced.