Something about how you go about instantiating Worlds has changed: When I try to create a new World using a different constructor the following error occurs: "An object with this name already exists."

Greenfoot UI: The class demarcations of World, Actor and Other are no longer visible. Unsure if this is intentional. There is white space between World and Actor and Other classes. I think this is useful for beginners.

Yes, that's a known issue, thanks. I've filed a bug ticket for this. That's an unavoidable change as a result of moving the interface to JavaFX, I'm afraid. We have less control over what is displayed in the dialogs and can't change the scenario folder icons to a Greenfoot icon. I'll look into this and see if I can reproduce. If you could provide more details (a specific example) would be great. It was not re-implemented in the new user interface. I think that's intentional, but can discuss it with the team. Thanks!

Never mind - this turns out to be trivial to reproduce. I've filed another ticket.

By the way: as it seems the same error occurs when an Actor subclass has a second constructor ("An object with this name already exists.").

Thanks, added to ticket.