_placeInTree(parentBranch, node, skipMassUpdate) {
    if (skipMassUpdate != true || skipMassUpdate === undefined) {
      // update the mass of the branch.
      this._updateBranchMass(parentBranch, node);
    }

    const range = parentBranch.children.NW.range;
    let region;
    if (range.maxX > node.x) {
      // in NW or SW
      if (range.maxY > node.y) {
        region = "NW";
      } else {
        region = "SW";
      }
    } else {
      // in NE or SE
      if (range.maxY > node.y) {
        region = "NE";
      } else {
        region = "SE";
      }
    }

    this._placeInRegion(parentBranch, node, region);
  }
determine in which branch the node will be placed.
@param {object} parentBranch
@param {Node} node
@param {boolean} skipMassUpdate
@private