The source code

    /*!
     * Ext JS Library 3.2.1
     * Copyright(c) 2006-2010 Ext JS, Inc.
     * licensing@extjs.com
     * http://www.extjs.com/license
     */
    /**
     * @class Ext.layout.ColumnLayout
     * @extends Ext.layout.ContainerLayout
     * This is the layout style of choice for creating structural layouts in a multi-column format where the width of
     * each column can be specified as a percentage or fixed width, but the height is allowed to vary based on the content.
     * This class is intended to be extended or created via the layout:'column' {@link Ext.Container#layout} config,
     * and should generally not need to be created directly via the new keyword.
     * ColumnLayout does not have any direct config options (other than inherited ones), but it does support a
     * specific config property of columnWidth that can be included in the config of any panel added to it.  The
     * layout will use the columnWidth (if present) or width of each panel during layout to determine how to size each panel.
     * If width or columnWidth is not specified for a given panel, its width will default to the panel's width (or auto).
     * The width property is always evaluated as pixels, and must be a number greater than or equal to 1.
     * The columnWidth property is always evaluated as a percentage, and must be a decimal value greater than 0 and
     * less than 1 (e.g., .25).
     * The basic rules for specifying column widths are pretty simple.  The logic makes two passes through the
     * set of contained panels.  During the first layout pass, all panels that either have a fixed width or none
     * specified (auto) are skipped, but their widths are subtracted from the overall container width.  During the second
     * pass, all panels with columnWidths are assigned pixel widths in proportion to their percentages based on
     * the total remaining container width.  In other words, percentage width panels are designed to fill the space
     * left over by all the fixed-width and/or auto-width panels.  Because of this, while you can specify any number of columns
     * with different percentages, the columnWidths must always add up to 1 (or 100%) when added together, otherwise your
     * layout may not render as expected.  Example usage:
     *
    // All columns are percentages -- they must add up to 1
    var p = new Ext.Panel({
        title: 'Column Layout - Percentage Only',
        layout:'column',
        items: [{
            title: 'Column 1',
            columnWidth: .25
        },{
            title: 'Column 2',
            columnWidth: .6
        },{
            title: 'Column 3',
            columnWidth: .15
        }]
    });

    // Mix of width and columnWidth -- all columnWidth values must add up
    // to 1. The first column will take up exactly 120px, and the last two
    // columns will fill the remaining container width.
    var p = new Ext.Panel({
        title: 'Column Layout - Mixed',
        layout:'column',
        items: [{
            title: 'Column 1',
            width: 120
        },{
            title: 'Column 2',
            columnWidth: .8
        },{
            title: 'Column 3',
            columnWidth: .2
        }]
    });

     */
    Ext.layout.ColumnLayout = Ext.extend(Ext.layout.ContainerLayout, {
        // private
        monitorResize:true,

        type: 'column',

        extraCls: 'x-column',

        scrollOffset : 0,

        // private

        targetCls: 'x-column-layout-ct',

        isValidParent : function(c, target){
            return this.innerCt && c.getPositionEl().dom.parentNode == this.innerCt.dom;
        },

        getLayoutTargetSize : function() {
            var target = this.container.getLayoutTarget(), ret;
            if (target) {
                ret = target.getViewSize();

                // IE in strict mode will return a width of 0 on the 1st pass of getViewSize.
                // Use getStyleSize to verify the 0 width, the adjustment pass will then work properly
                // with getViewSize
                if (Ext.isIE && Ext.isStrict && ret.width == 0){
                    ret =  target.getStyleSize();
                }

                ret.width -= target.getPadding('lr');
                ret.height -= target.getPadding('tb');
            }
            return ret;
        },

        renderAll : function(ct, target) {
            if(!this.innerCt){
                // the innerCt prevents wrapping and shuffling while
                // the container is resizing
                this.innerCt = target.createChild({cls:'x-column-inner'});
                this.innerCt.createChild({cls:'x-clear'});
            }
            Ext.layout.ColumnLayout.superclass.renderAll.call(this, ct, this.innerCt);
        },

        // private
        onLayout : function(ct, target){
            var cs = ct.items.items,
                len = cs.length,
                c,
                i,
                m,
                margins = [];

            this.renderAll(ct, target);

            var size = this.getLayoutTargetSize();

            if(size.width < 1 && size.height < 1){ // display none?
                return;
            }

            var w = size.width - this.scrollOffset,
                h = size.height,
                pw = w;

            this.innerCt.setWidth(w);

            // some columns can be percentages while others are fixed
            // so we need to make 2 passes

            for(i = 0; i < len; i++){
                c = cs[i];
                m = c.getPositionEl().getMargins('lr');
                margins[i] = m;
                if(!c.columnWidth){
                    pw -= (c.getWidth() + m);
                }
            }

            pw = pw < 0 ? 0 : pw;

            for(i = 0; i < len; i++){
                c = cs[i];
                m = margins[i];
                if(c.columnWidth){
                    c.setSize(Math.floor(c.columnWidth * pw) - m);
                }
            }

            // Browsers differ as to when they account for scrollbars.  We need to re-measure to see if the scrollbar
            // spaces were accounted for properly.  If not, re-layout.
            if (Ext.isIE) {
                if (i = target.getStyle('overflow') && i != 'hidden' && !this.adjustmentPass) {
                    var ts = this.getLayoutTargetSize();
                    if (ts.width != size.width){
                        this.adjustmentPass = true;
                        this.onLayout(ct, target);
                    }
                }
            }
            delete this.adjustmentPass;
        }

        /**
         * @property activeItem
         * @hide
         */
    });

    Ext.Container.LAYOUTS['column'] = Ext.layout.ColumnLayout;