constructor (parent) {
    super(parent)
    // Config:
    this.activeUpAngle = Math.PI / 7
    this.preferredHand = 'left'
    this.platformRadius = 0.25
    this.platformColor = 0x333333
    this.projectionColor = 0x3399ff
    this.keepContentAlive = false
    this.onActiveChange = null

    // Internal state:
    this.gripPose = null
    this.active = false

    this._cogPos = new Vector3()
    this.addEventListener('xrframe', this.onXRFrame.bind(this))
  }
This facade provides a container for arbitrary global UI, which is hidden by default
but is easily brought up by a simple gesture. A wristband is added to one of the hand
controllers, with an icon affordance on the inner wrist. When that icon is turned
upwards, the UI is projected from it.

@property {('left'|'right')} preferredHand - which hand the wristband should appear on.