Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding Troubleshooting Broken Camera in Editor.

Found the relevant reddit post:

https://www.reddit.com/r/DivinityOriginalSin/comments/74teof/tech_help_cant_rotate_camera_in_the_divinity/

Holding RMB at the same time as the MMB does let me look around at least, even if it's still not optimal.