is.target_) return;
    this.observer_.observe(this.target_, {
      attributes: true,
      attributeFilter: ['class']
    });
  },

  get horizontal() {
    return this.horizontal_;
  },

  set horizontal(h) {
    this.horizontal_ = h;
    if (this.horizontal_) {
      this.className = 'horizontal-drag-handle';
    } else {
      this.className = 'vertical-drag-handle';
    }
  },

  get vertical() {
    return !this.horizontal_;
  },

  set vertical(v) {
    this.horizontal = !v;
  },

  forceMutationObs