If you’re running Unity under Xubuntu Linux (20.04) you might find that Alt + Left Mouse button resizes Unity and drags the whole window around rather than rotating around the object.
Alt is used by Xubuntu as an Accessibility key for grabbing and moving windows. To change this behaviour find/type Window Manager Tweaks into the whisker menu and run the program. A Window Manager Tweaks window should open.
Click the Accessibility tab. At the top of the Accessibility tab is an option Key used to grab and move windows. Change this from Alt to None (if you need Accessibility settings you can always change this to one of the other keys listed). You should now be able to rotate around the object in Unity.
I’m just starting my Unity journey. I know this probably won’t be useful to many of you, but hopefully this helps someone out there.