nst delta = this.lastMousePos_ - curMousePos;

    this.applyDelta_(delta);

    this.lastMousePos_ = curMousePos;
    e.preventDefault();
    return true;
  },

  onMouseDown_(e) {
    if (!this.target_) return;
    this.forceMutationObserverFlush_();
    // Start with the current actual size.
    this.currentDraggingSize_ = this.getTargetSize_();
    this.lastMousePos_ = this.horizontal_ ? e.clientY : e.clientX;
    document.addEventListener('mousemove', this.onMouseMove_);
    document.addEventListener('