After spending quite some time delving into the code of this Trackball control, I am puzzled by what exactly this.object.position represents. You can find the code here: https://github.com/mrdoob/three.js/blob/master/examples/js/controls/TrackballControls.js
The author not only stores the original value of position (for resetting purposes) but also keeps track of the lastPosition whenever any changes are made to the position after each frame.
... // initialization code
var lastPosition = new THREE.Vector3();
... // more setup
// for reset
this.position0 = this.object.position.clone();
Initially, I presumed that it refers to the camera's position, but there's also a 'this.eye' variable which could cause confusion. Then I considered if it relates to the drawing surface's position on the DOM, but since it's a size 3 vector, this theory doesn't seem plausible either.
Admittedly, this may sound like a simple question, but I would genuinely appreciate any guidance in the right direction.
Thank you for taking the time to read this.