Ah, I see. It's declared outside of the function scope only to avoid array allocations. Because it was declared at the outer scope, I mistook it for part of the control's state. But other code can perfectly well change the camera's position between `OrbitControl` updates, in which case `offset` would have a bogus value.

Removed in lpsinger/three.js@ce1554136404e35b22a5af814ebb9fd8b4a7fcd8.