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

[Stefan Monnier]

>> AFAIK if the buffer has not been modified (including things like
>> changing `window-start` or `point`), then a redisplay will just not
>> run jit-lock (and hence font-lock) at all, no matter how thorough.
> But the fact is without font-lock the response is faster by a large
> factor.  So something, somewhere, still depends on font-lock.

Yes, that's the part that we need to explore.
Maybe font-lock *is* run somehow?
Or maybe it's just the mere presence of text properties?  (Or overlays?)


        Stefan