    /**
     * @private
     */
    Ext.define('Ext.grid.header.DropZone', {
        extend: 'Ext.dd.DropZone',
        colHeaderCls: Ext.baseCSSPrefix + 'column-header',
        proxyOffsets: [-4, -9],

        constructor: function(headerCt){
            this.headerCt = headerCt;
            this.ddGroup = this.getDDGroup();
            this.callParent([headerCt.el]);
        },

        getDDGroup: function() {
            return 'header-dd-zone-' + this.headerCt.up('[scrollerOwner]').id;
        },

        getTargetFromEvent : function(e){
            return e.getTarget('.' + this.colHeaderCls);
        },

        getTopIndicator: function() {
            if (!this.topIndicator) {
                this.topIndicator = Ext.DomHelper.append(Ext.getBody(), {
                    cls: "col-move-top",
                    html: "&#160;"
                }, true);
            }
            return this.topIndicator;
        },

        getBottomIndicator: function() {
            if (!this.bottomIndicator) {
                this.bottomIndicator = Ext.DomHelper.append(Ext.getBody(), {
                    cls: "col-move-bottom",
                    html: "&#160;"
                }, true);
            }
            return this.bottomIndicator;
        },

        getLocation: function(e, t) {
            var x      = e.getXY()[0],
                region = Ext.fly(t).getRegion(),
                pos, header;

            if ((region.right - x) <= (region.right - region.left) / 2) {
                pos = "after";
            } else {
                pos = "before";
            }
            return {
                pos: pos,
                header: Ext.getCmp(t.id),
                node: t
            };
        },

        positionIndicator: function(draggedHeader, node, e){
            var location = this.getLocation(e, node),
                header = location.header,
                pos    = location.pos,
                nextHd = draggedHeader.nextSibling('gridcolumn:not([hidden])'),
                prevHd = draggedHeader.previousSibling('gridcolumn:not([hidden])'),
                topIndicator, bottomIndicator, topAnchor, bottomAnchor,
                topXY, bottomXY, headerCtEl, minX, maxX,
                allDropZones, ln, i, dropZone;

            // Cannot drag beyond non-draggable start column
            if (!header.draggable && header.getIndex() === 0) {
                return false;
            }

            this.lastLocation = location;

            if ((draggedHeader !== header) &&
                ((pos === "before" && nextHd !== header) ||
                (pos === "after" && prevHd !== header)) &&
                !header.isDescendantOf(draggedHeader)) {

                // As we move in between different DropZones that are in the same
                // group (such as the case when in a locked grid), invalidateDrop
                // on the other dropZones.
                allDropZones = Ext.dd.DragDropManager.getRelated(this);
                ln = allDropZones.length;
                i  = 0;

                for (; i < ln; i++) {
                    dropZone = allDropZones[i];
                    if (dropZone !== this && dropZone.invalidateDrop) {
                        dropZone.invalidateDrop();
                    }
                }

                this.valid = true;
                topIndicator = this.getTopIndicator();
                bottomIndicator = this.getBottomIndicator();
                if (pos === 'before') {
                    topAnchor = 'tl';
                    bottomAnchor = 'bl';
                } else {
                    topAnchor = 'tr';
                    bottomAnchor = 'br';
                }
                topXY = header.el.getAnchorXY(topAnchor);
                bottomXY = header.el.getAnchorXY(bottomAnchor);

                // constrain the indicators to the viewable section
                headerCtEl = this.headerCt.el;
                minX = headerCtEl.getLeft();
                maxX = headerCtEl.getRight();

                topXY[0] = Ext.Number.constrain(topXY[0], minX, maxX);
                bottomXY[0] = Ext.Number.constrain(bottomXY[0], minX, maxX);

                // adjust by offsets, this is to center the arrows so that they point
                // at the split point
                topXY[0] -= 4;
                topXY[1] -= 9;
                bottomXY[0] -= 4;

                // position and show indicators
                topIndicator.setXY(topXY);
                bottomIndicator.setXY(bottomXY);
                topIndicator.show();
                bottomIndicator.show();
            // invalidate drop operation and hide indicators
            } else {
                this.invalidateDrop();
            }
        },

        invalidateDrop: function() {
            this.valid = false;
            this.hideIndicators();
        },

        onNodeOver: function(node, dragZone, e, data) {
            var me = this,
                header = me.headerCt,
                doPosition = true,
                from = data.header,
                to;

            if (data.header.el.dom === node) {
                doPosition = false;
            } else {
                to = me.getLocation(e, node).header;
                doPosition = (from.ownerCt === to.ownerCt) || (!from.ownerCt.sealed && !to.ownerCt.sealed);
            }

            if (doPosition) {
                me.positionIndicator(data.header, node, e);
            } else {
                me.valid = false;
            }
            return me.valid ? me.dropAllowed : me.dropNotAllowed;
        },

        hideIndicators: function() {
            this.getTopIndicator().hide();
            this.getBottomIndicator().hide();
        },

        onNodeOut: function() {
            this.hideIndicators();
        },

        onNodeDrop: function(node, dragZone, e, data) {
            if (this.valid) {
                var dragHeader   = data.header,
                    lastLocation = this.lastLocation,
                    targetHeader = lastLocation.header,
                    fromCt       = dragHeader.ownerCt,
                    fromHeader   = dragHeader.up('headercontainer:not(gridcolumn)'),
                    localFromIdx = fromCt.items.indexOf(dragHeader), // Container.items is a MixedCollection
                    toCt         = targetHeader.ownerCt,
                    toHeader     = targetHeader.up('headercontainer:not(gridcolumn)'),
                    localToIdx   = toCt.items.indexOf(targetHeader),
                    headerCt     = this.headerCt,
                    fromIdx      = headerCt.getHeaderIndex(dragHeader),
                    colsToMove   = dragHeader.isGroupHeader ? dragHeader.query(':not([isGroupHeader])').length : 1,
                    toIdx        = headerCt.getHeaderIndex(targetHeader),
                    groupCt,
                    scrollerOwner;

                // Drop position is to the right of the targetHeader, increment the toIdx correctly
                if (lastLocation.pos === 'after') {
                    localToIdx++;
                    toIdx += targetHeader.isGroupHeader ? targetHeader.query(':not([isGroupHeader])').length : 1;
                }

                // If we are dragging in between two HeaderContainers that have had the lockable
                // mixin injected we will lock/unlock headers in between sections, and then continue
                // with another execution of onNodeDrop to ensure the header is dropped into the correct group
                if (fromHeader !== toHeader && fromHeader.lockableInjected && toHeader.lockableInjected && toHeader.lockedCt) {
                    scrollerOwner = fromCt.up('[scrollerOwner]');
                    scrollerOwner.lock(dragHeader, localToIdx);

                    // Now that the header has been transferred into the correct HeaderContainer, recurse, and continue the drop operation with the same dragData
                    this.onNodeDrop(node, dragZone, e, data);
                } else if (fromHeader !== toHeader && fromHeader.lockableInjected && toHeader.lockableInjected && fromHeader.lockedCt) {
                    scrollerOwner = fromCt.up('[scrollerOwner]');
                    scrollerOwner.unlock(dragHeader, localToIdx);

                    // Now that the header has been transferred into the correct HeaderContainer, recurse, and continue the drop operation with the same dragData
                    this.onNodeDrop(node, dragZone, e, data);
                }

                // This is a drop within the same HeaderContainer.
                else {
                    this.invalidateDrop();

                    // If dragging rightwards, then after removal, the insertion index will be less when moving
                    // within the same container.
                    if ((fromCt === toCt) && (localToIdx > localFromIdx)) {

                        // Wer're dragging whole headers, so locally, the adjustment is only one
                        localToIdx -= 1;
                    }

                    // Suspend layouts while we sort all this out.
                    Ext.suspendLayouts();

                    // Remove dragged header from where it was.
                    if (fromCt !== toCt) {
                        fromCt.remove(dragHeader, false);

                        // Dragged the last header out of the fromCt group... The fromCt group must die
                        if (fromCt.isGroupHeader) {
                            if (!fromCt.items.getCount()) {
                                groupCt = fromCt.ownerCt;
                                groupCt.remove(fromCt, false);
                                fromCt.el.dom.parentNode.removeChild(fromCt.el.dom);
                            }
                        }
                    }

                    // Move dragged header into its drop position
                    if (fromCt === toCt) {
                        toCt.move(localFromIdx, localToIdx);
                    } else {
                        toCt.insert(localToIdx, dragHeader);
                    }

                    // Group headers acquire the aggregate width of their child headers
                    // Therefore a child header may not flex; it must contribute a fixed width.
                    // But we restore the flex value when moving back into the main header container
                    if (toCt.isGroupHeader) {
                        // Adjust the width of the "to" group header only if we dragged in from somewhere else.
                        if (toCt !== fromCt) {
                            dragHeader.savedFlex = dragHeader.flex;
                            delete dragHeader.flex;
                            dragHeader.width = dragHeader.getWidth();
                        }
                    } else {
                        if (dragHeader.savedFlex) {
                            dragHeader.flex = dragHeader.savedFlex;
                            delete dragHeader.width;
                        }
                    }

                    // Refresh columns cache in case we remove an emptied group column
                    headerCt.purgeCache();
                    Ext.resumeLayouts(true);
                    headerCt.onHeaderMoved(dragHeader, colsToMove, fromIdx, toIdx);

                    // Emptied group header can only be destroyed after the header and grid have been refreshed
                    if (!fromCt.items.getCount()) {
                        fromCt.destroy();
                    }
                }
            }
        }
    });