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

[Gregory Heytings]

> > If and when it is solved, we could revisit this issue.
>
> The locked narrowing currently in effect makes it impossible to 
> investigate this problem or improve the behavior :-(

How so?  It is fully backward compatible, just (setq long-line-threshold 
nil).  If what you want is only to remove the locked narrowing, comment 
out the relevant line in xdisp.c.