Thanks - it seems to be a graphics bug - when Cubase is open on my small screened Macboob pro - it needs to be clicked on a few time to reveal the partitions - though I can’t resize them.
But:
If I put the project window on a larger second monitor and resize the whole project window - even by a small amount - It behaves correctly.
Thanks - I did a re-install - same behaviour.
I’ll report it.