onTouchMove_(event) {
    if (event.touches.length > 0 &&
        this.touchStartX_ != null &&
        this.touchStartY_ != null) {
      const dx = event.touches[0].clientX - this.touchStartX_;
      const dy = event.touches[0].clientY - this.touchStartY_;
      const distance = Math.sqrt(dx * dx + dy * dy);
      if (distance > shaka.ui.HiddenSeekButton.SCROLL_THRESHOLD_) {
        this.hasMoved_ = true;
      }
    }
  }
Called when the user moves the finger on the screen.
If the movement exceeds the scroll threshold, we mark this as scrolling.
@param {!TouchEvent} event
@private