This seems to have arisen out of nowhere, but Im sure Ive changed something!!

I can't drag my canvas from my iMac (M1) to my Viewsonic VX3211-4K. All the tool panels etc can be dragged as normal. iMac is set as Main Display and Viewsonic as Extended display. I have not upgraded MacOs, I have restarted twice. Other software such as Rebelle, Affinity apps are all working normally.

It's a mystery - at least to me!

Any ideas welcome - please.