bug#56682: locked narrowing

[Gregory Heytings]

> We only need to know whether there are long lines in a region around 
> point that is large enough to make sure redisplay won't look beyond 
> that.

Sorry, but no, we don't.

> (Gregory disagrees, but I have yet to see a reason why he would be right 
> and I would be wrong in this matter.)

Consider this other scenario.  A buffer is buried, and modified in the 
background by some process.  That process adds a long line in that buffer, 
followed by enough non-long lines, with point after these non-long lines. 
Now the user asks for that buffer to be displayed again.  With your 
proposed approach, we would search around point if there are long lines. 
There aren't, so we do not activate long line optimizations.  Now the user 
scrolls the buffer.  The buffer has not changed in any way, so the 
detection is not triggered, and we reach the long line.  Kaboom!