If anybody ever has the energy to look at the redisplay routine

[Earl A. Killian <EAK @ MIT-MC>]

I believe this is a "feature".  It is considered too expensive to
update the hash code of a line for operations such as
self-insert, and so it just sets the hash code to -1.  The next
redisplay that specifies bounds that include that line will
retype it, because the hash code is wrong.  The reason that it
doesn't retyped more often is that most things are so good about
returning accurate buffer bounds to limit redisplay.