Now you can turn on Arcball Rotate in Engine Preferences, which is cool.
But it does not work like Arcball Rotate which most 3D apps and engines already have.
It is fine that you can only initiate this operation from within this red triangle area:
But the rotation just stops when you hit the edges of this area, which breaks the convention of this tool. Arcball rotate is a well established artist tool for scene editing. I do hope it will eventually work as expected.
Cheers!