bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Gregory Heytings]

> Gregory, I suspect that the problem could be in the loop in 
> redisplay_window, where we look for long lines.  Changing visibility in 
> Org changes text properties, and that causes MODIFF to be incremented. 
> I actually can cause something similar without Org at all, just by 
> scrolling fast through xdisp.c.  You can put a breakpoint inside the 
> 'if' that guards the re-evaluation of the long-lines in 
> redisplay_window, and scroll with C-v through xdisp.c immediately after 
> visiting it -- I hit the breakpoint from time to time, although there 
> are no changes to the buffer except faces placed by fontifications.
>
> jit-lock runs under with-silent-modifications, but that only adjusts 
> SAVE_MODIFF to create an illusion of unchanged buffer, it doesn't affect 
> UNCHANGED_MODIFIED.

Indeed, that's problematic: modify_text_properties calls modiff_incr 
(&MODIFF, 1).  I think the safest change would be to use CHARS_MODIFF 
instead of MODIFF, see attached.

Ihor, can you please test that patch and tell us if it solves your 
problem?

use_chars_modiff.patch
Description: Text Data