bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

> I think we should simply unconditionally recompute the narrowing in 
> 'reseat'.  At least I couldn't think of a situation where that would 
> cause trouble, and 'reseat' is called rarely enough not to make this 
> expensive.  Am I missing something?

I don't think you are missing something, and that's what I suggested too. 
So there is nothing to change here, right?

> And another nit:
>
>  if (current_buffer->long_line_optimizations_p)
>    {
>      if (!it->narrowed_begv
>           || ((pos.charpos < it->narrowed_begv || pos.charpos > it->narrowed_zv)
>               && (!redisplaying_p || it->line_wrap == TRUNCATE)))
>         {
>           it->narrowed_begv = get_narrowed_begv (it->w, window_point (it->w));
>           it->narrowed_zv = get_narrowed_zv (it->w, window_point (it->w));
>         }
>    }
>
> I think this should pass pos.charpos as the 2nd argument to 
> get_narrowed_begv and get_narrowed_zv, otherwise it might not really 
> correct anything, right?  In particular, when lines are truncated, that 
> will definitely happen when we display any line but the very first.
>
> Or perhaps we should check that using window-point indeed brings 
> pos.charpos into the narrowed region, and only use pos.charpos if it 
> doesn't?

I changed this into:

  if (current_buffer->long_line_optimizations_p)
    {
      if (!it->narrowed_begv)
        {
          it->narrowed_begv = get_narrowed_begv (it->w, window_point (it->w));
          it->narrowed_zv = get_narrowed_zv (it->w, window_point (it->w));
        }
      else if ((pos.charpos < it->narrowed_begv || pos.charpos > 
it->narrowed_zv)
                && (!redisplaying_p || it->line_wrap == TRUNCATE))
        {
          it->narrowed_begv = get_narrowed_begv (it->w, pos.charpos);
          it->narrowed_zv = get_narrowed_zv (it->w, pos.charpos);
        }
    }

which seems better indeed.  Is that okay from your point of view?