    /**
     * Private utility class for Ext.BorderSplitter.
     * @private
     */
    Ext.define('Ext.resizer.BorderSplitterTracker', {
        extend: 'Ext.resizer.SplitterTracker',
        requires: ['Ext.util.Region'],

        getPrevCmp: null,
        getNextCmp: null,

        // calculate the constrain Region in which the splitter el may be moved.
        calculateConstrainRegion: function() {
            var me = this,
                splitter = me.splitter,
                collapseTarget = splitter.collapseTarget,
                defaultSplitMin = splitter.defaultSplitMin,
                sizePropCap = splitter.vertical ? 'Width' : 'Height',
                minSizeProp = 'min' + sizePropCap,
                maxSizeProp = 'max' + sizePropCap,
                getSizeMethod = 'get' + sizePropCap,
                neighbors = splitter.neighbors,
                length = neighbors.length,
                box = collapseTarget.el.getBox(),
                left = box.x,
                top = box.y,
                right = box.right,
                bottom = box.bottom,
                size = splitter.vertical ? (right - left) : (bottom - top),
                //neighborSizes = [],
                i, neighbor, minRange, maxRange, maxGrowth, maxShrink, targetSize;

            // if size=100 and minSize=80, we can reduce by 20 so minRange = minSize-size = -20
            minRange = (collapseTarget[minSizeProp] || Math.min(size,defaultSplitMin)) - size;

            // if maxSize=150, maxRange = maxSize - size = 50
            maxRange = collapseTarget[maxSizeProp];
            if (!maxRange) {
                maxRange = 1e9;
            } else {
                maxRange -= size;
            }
            targetSize = size;

            for (i = 0; i < length; ++i) {
                neighbor = neighbors[i];
                size = neighbor[getSizeMethod]();
                //neighborSizes.push(size);

                maxGrowth = size - neighbor[maxSizeProp]; // NaN if no maxSize or negative
                maxShrink = size - (neighbor[minSizeProp] || Math.min(size,defaultSplitMin));

                if (!isNaN(maxGrowth)) {
                    // if neighbor can only grow by 10 (maxGrowth = -10), minRange cannot be
                    // -20 anymore, but now only -10:
                    if (minRange < maxGrowth) {
                        minRange = maxGrowth;
                    }
                }

                // if neighbor can shrink by 20 (maxShrink=20), maxRange cannot be 50 anymore,
                // but now only 20:
                if (maxRange > maxShrink) {
                    maxRange = maxShrink;
                }
            }

            if (maxRange - minRange < 2) {
                return null;
            }

            box = new Ext.util.Region(top, right, bottom, left);

            me.constraintAdjusters[me.getCollapseDirection()](box, minRange, maxRange, splitter);

            me.dragInfo = {
                minRange: minRange,
                maxRange: maxRange,
                //neighborSizes: neighborSizes,
                targetSize: targetSize
            };

            return box;
        },

        constraintAdjusters: {
            // splitter is to the right of the box
            left: function (box, minRange, maxRange, splitter) {
                box[0] = box.x = box.left = box.right + minRange;
                box.right += maxRange + splitter.getWidth();
            },

            // splitter is below the box
            top: function (box, minRange, maxRange, splitter) {
                box[1] = box.y = box.top = box.bottom + minRange;
                box.bottom += maxRange + splitter.getHeight();
            },

            // splitter is above the box
            bottom: function (box, minRange, maxRange, splitter) {
                box.bottom = box.top - minRange;
                box.top -= maxRange + splitter.getHeight();
            },

            // splitter is to the left of the box
            right: function (box, minRange, maxRange, splitter) {
                box.right = box.left - minRange;
                box[0] = box.x = box.left = box.x - maxRange + splitter.getWidth();
            }
        },

        onBeforeStart: function(e) {
            var me = this,
                splitter = me.splitter,
                collapseTarget = splitter.collapseTarget,
                neighbors = splitter.neighbors,
                collapseEl = me.getSplitter().collapseEl,
                target = e.getTarget(),
                length = neighbors.length,
                i, neighbor;

            if (collapseEl && target === splitter.collapseEl.dom) {
                return false;
            }

            if (collapseTarget.collapsed) {
                return false;
            }

            // disabled if any neighbors are collapsed in parallel direction.
            for (i = 0; i < length; ++i) {
                neighbor = neighbors[i];

                if (neighbor.collapsed && neighbor.isHorz === collapseTarget.isHorz) {
                    return false;
                }
            }

            if (!(me.constrainTo = me.calculateConstrainRegion())) {
                return false;
            }

            return true;
        },

        performResize: function(e, offset) {
            var me = this,
                splitter = me.splitter,
                collapseDirection = splitter.getCollapseDirection(),
                collapseTarget = splitter.collapseTarget,
                // a vertical splitter adjusts horizontal dimensions
                adjusters = me.splitAdjusters[splitter.vertical ? 'horz' : 'vert'],
                delta = offset[adjusters.index],
                dragInfo = me.dragInfo,
                //neighbors = splitter.neighbors,
                //length = neighbors.length,
                //neighborSizes = dragInfo.neighborSizes,
                //isVert = collapseTarget.isVert,
                //i, neighbor,
                owner;

            if (collapseDirection == 'right' || collapseDirection == 'bottom') {
                // these splitters grow by moving left/up, so flip the sign of delta...
                delta = -delta;
            }

            // now constrain delta to our computed range:
            delta = Math.min(Math.max(dragInfo.minRange, delta), dragInfo.maxRange);

            if (delta) {
                (owner = splitter.ownerCt).suspendLayouts();

                adjusters.adjustTarget(collapseTarget, dragInfo.targetSize, delta);

                //for (i = 0; i < length; ++i) {
                //    neighbor = neighbors[i];
                //    if (!neighbor.isCenter && !neighbor.maintainFlex && neighbor.isVert == isVert) {
                //        delete neighbor.flex;
                //        adjusters.adjustNeighbor(neighbor, neighborSizes[i], delta);
                //    }
                //}

                owner.resumeLayouts(true);
            }
        },

        splitAdjusters: {
            horz: {
                index: 0,
                //adjustNeighbor: function (neighbor, size, delta) {
                //    neighbor.setSize(size - delta);
                //},
                adjustTarget: function (target, size, delta) {
                    target.flex = null;
                    target.setSize(size + delta);
                }
            },
            vert: {
                index: 1,
                //adjustNeighbor: function (neighbor, size, delta) {
                //    neighbor.setSize(undefined, size - delta);
                //},
                adjustTarget: function (target, targetSize, delta) {
                    target.flex = null;
                    target.setSize(undefined, targetSize + delta);
                }
            }
        },

        getCollapseDirection: function() {
            return this.splitter.getCollapseDirection();
        }
    });