some goop into the crack which makes it invisible and stops it spreading.

Yeah, they do. Unfortunately, I think that cracks above a certain size will require replacement. In the UK, at least, I think a cracked windscreen will cause you to fail your MOT (roadworthiness test -- once a year).

Fortunately, most UK insurance policies don't incur a no-claims penalty for glass replacement -- although you still have to pay the excess -- as I found out the other week when some [censored] broke into my car. Fortunately, there was nothing in it, but I had to replace the driver-side rear window, and bend the driver's door frame back into shape where they'd tried to force it.

And I've got to take the car for MOT this month, which is going to cost more money. Cars, eh?