code: function (evt) {
      if (!this.data.enabled) { return; }
      // Handle only primary button.
      if (evt.button !== 0) { return; }
  
      var sceneEl = this.el.sceneEl;
      var canvasEl = sceneEl && sceneEl.canvas;
  
      this.mouseDown = true;
      this.previousMouseEvent = evt;
      this.showGrabbingCursor();
  
      if (this.data.pointerLockEnabled && !this.pointerLocked) {
        if (canvasEl.requestPointerLock) {
          canvasEl.requestPointerLock();
        } else if (canvasEl.mozRequestPointerLock) {
          canvasEl.mozRequestPointerLock();
        }
      }
    }
docs: Register mouse down to detect mouse drag.