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

[Gregory Heytings]

> And I don't think it's something you should be ashamed of: it's standard 
> practice to use expedient solutions for want of anything better.  It's 
> how we make progress.
>
> Those expedient solutions (i.e. those cheap solutions that still cover 
> most cases tolerably) might be "not ideal" but finding them is usually 
> not easy, so pride would be more appropriate.

I'm neither ashamed nor proud, I'm not Kim Kardashian 😉  I just try to 
have fun, for the programmer-specific value of "fun".  Still, I cannot see 
how an editor could provide a complete editing experience in all buffers 
without limiting in one way or another what is done, and what can be done, 
after each editing command.