Text entry box

A really annoying thing has happened recently with my Sony Xperia z5 on Android 7.1.1. First noticed it in whatsapp but also happens e.g. with messaging app. When I click on the box to enter text it goes to a full screen text entry box instead of showing the message as I type in the context of other messages. I guess there is a setting somewhere but I've searched everywhere and not been able to find anything.

Any ideas?

Thanks!

Which keyboard? They usually have resize options.