Stefan Monnier wrote:
> FWIW, I recommend against using
> `font-lock-extend-after-change-region-function`.
> 
> You can use `font-lock-extend-region-functions` instead (which performs
> the region-extension right before fontifying a chunk) to avoid this problem.
> [ It won't help with the current performance issue, tho.  ]

Thank you for your advice.  Does the attached patch seem reasonable?