@mikel @rateldajer Replying to a CW toot copies the CW text.
@mikel I think it's to prevent, say, "CW: Doctor Who spoilers: Kris Marshall is playing the Doctor's mum." getting un-CW'd responses -- it works very well in this specific way, but I imagine that means it's probably not very useful for that many other uses. (I'd rename the feature "spoiler" over "CW".)
@Daily Thanks - it took me by surprise to be honest. I don't find that to be very intuitive. It copied the non-hidden text, and when you click "show more" it shows my intended reply