    /**
     * @private
     *
     * Manages and provides information about a TablePanel's *visible leaf* columns.
     */
    Ext.define('Ext.grid.ColumnManager', {
        alternateClassName: ['Ext.grid.ColumnModel'],

        columns: null,

        constructor: function(visibleOnly, headerCt, secondHeaderCt) {
            //<debug>
            if (!headerCt.isRootHeader && !headerCt.isGroupHeader) {
                Ext.Error.raise('ColumnManager must be passed an instantiated HeaderContainer or group header');
            }
            //</debug>
            this.headerCt = headerCt;

            // We are managing columns for a lockable grid...
            if (secondHeaderCt) {
                //<debug>
                if (!headerCt.isRootHeader && !headerCt.isGroupHeader) {
                    Ext.Error.raise('ColumnManager must be passed an instantiated HeaderContainer or group header');
                }
                //</debug>
                this.secondHeaderCt = secondHeaderCt;
            }
            this.visibleOnly = !!visibleOnly;
        },

        getColumns: function() {
            if (!this.columns) {
                this.cacheColumns();
            }
            return this.columns;
        },

        /**
         * If called from a root header, returns the index of a leaf level header regardless of what the nesting
         * structure is.
         *
         * If called from a group header, returns the index of a leaf level header relative to the group header.
         *
         * If a group header is passed, the index of the first leaf level header within it is returned.
         *
         * @param {Ext.grid.column.Column} header The header to find the index of
         * @return {Number} The index of the specified column header
         */
        getHeaderIndex: function (header) {
            if (header.isGroupHeader) {
                // Get the first header for the particular group header. The .getHeaderColumns API
                // will sort out if it's to be just visible columns or all columns.
                header = this.getHeaderColumns(header)[0];
            }

            return Ext.Array.indexOf(this.getColumns(), header);
        },

        /**
         * If called from a root header, gets a leaf level header by index regardless of what the nesting
         * structure is.
         *
         * If called from a group header, returns the index of a leaf level header relative to the group header.
         *
         * @param {Number} index The column index for which to retrieve the column.
         * @return {Ext.grid.column.Column} The header. `null` if it doesn't exist.
         */
        getHeaderAtIndex: function(index) {
            var columns = this.getColumns(),
                col = columns[index];

            return col || null;
        },

        getPreviousSibling: function(header){
            var index = this.getHeaderIndex(header),
                col = null;

            if (index > 0) {
                col = this.getColumns()[index - 1];
            }
            return col;
        },

        getNextSibling: function(header){
            var index = this.getHeaderIndex(header),
                col;

            if (index !== -1) {
                col = this.getColumns()[index + 1];
            }
            return col || null;
        },

        /**
         * Get the first column.
         * @return {Ext.grid.column.Column} The header. `null` if it doesn't exist
         */
        getFirst: function(){
            var columns = this.getColumns();
            return columns.length > 0 ? columns[0] : null;
        },

        /**
         * Get the last column.
         * @return {Ext.grid.column.Column} The header. `null` if it doesn't exist
         */
        getLast: function(){
            var columns = this.getColumns(),
                len = columns.length;

            return len > 0 ? columns[len - 1] : null;
        },

        /**
         * Get a leaf level header by index regardless of what the nesting
         * structure is.
         * @param {String} id The id
         * @return {Ext.grid.column.Column} The header. `null` if it doesn't exist.
         */
        getHeaderById: function(id) {
            var columns = this.getColumns(),
                len = columns.length,
                i, header;

            for (i = 0; i < len; ++i) {
                header = columns[i];
                if (header.getItemId() === id) {
                    return header;
                }
            }
            return null;
        },

        /**
         * When passed a column index, returns the closet *visible* column to that. If the column at the passed index is visible,
         * that is returned. If it is hidden, either the next visible, or the previous visible column is returned.
         *
         * If called from a group header, returns the visible index of a leaf level header relative to the group header with the
         * same stipulations as outlined above.
         *
         * @param {Number} index Position at which to find the closest visible column.
         */
        getVisibleHeaderClosestToIndex: function(index) {
            var result = this.getHeaderAtIndex(index);
            if (result && result.hidden) {
                result = result.next(':not([hidden])') || result.prev(':not([hidden])');
            }
            return result;
        },

        cacheColumns: function() {
            var columns = this.getHeaderColumns(this.headerCt),
                second = this.secondHeaderCt;

            if (second) {
                columns = columns.concat(this.getHeaderColumns(second));
            }
            this.columns = columns;
        },

        getHeaderColumns: function(header) {
            var result = this.visibleOnly ? header.getVisibleGridColumns() : header.getGridColumns();
            return Ext.Array.clone(result);
        },

        invalidate: function() {
            var root = this.rootColumns;
            this.columns = null;

            // If we are part of a lockable assembly, invalidate the root column manager
            if (root) {
                root.invalidate();
            }
        },

        destroy: function(){
            this.columns = this.rootColumns = null;
        }
    }, function() {
        this.createAlias('indexOf', 'getHeaderIndex');
    });