The source code

    /*!
     * Ext JS Library 3.4.0
     * Copyright(c) 2006-2011 Sencha Inc.
     * licensing@sencha.com
     * http://www.sencha.com/license
     */
    /**
     * @class Ext.layout.HBoxLayout
     * @extends Ext.layout.BoxLayout
     * A layout that arranges items horizontally across a Container. This layout optionally divides available horizontal
     * space between child items containing a numeric flex configuration.
     * This layout may also be used to set the heights of child items by configuring it with the {@link #align} option.
     */
    Ext.layout.HBoxLayout = Ext.extend(Ext.layout.BoxLayout, {
        /**
         * @cfg {String} align
         * Controls how the child items of the container are aligned. Acceptable configuration values for this
         * property are:
         *
         * top : Defaultchild items are aligned vertically
         * at the top of the container
         * middle : child items are aligned vertically in the
         * middle of the container
         * stretch : child items are stretched vertically to fill
         * the height of the container
         * stretchmax : child items are stretched vertically to
         * the height of the largest item.
         */
        align: 'top', // top, middle, stretch, strechmax

        type : 'hbox',

        /**
         * @cfg {String} pack
         * Controls how the child items of the container are packed together. Acceptable configuration values
         * for this property are:
         *
         * start : Defaultchild items are packed together at
         * left side of container
         * center : child items are packed together at
         * mid-width of container
         * end : child items are packed together at right
         * side of container
         *
         */
        /**
         * @cfg {Number} flex
         * This configuation option is to be applied to child items of the container managed
         * by this layout. Each child item with a flex property will be flexed horizontally
         * according to each item's relative flex value compared to the sum of all items with
         * a flex value specified.  Any child items that have either a flex = 0 or
         * flex = undefined will not be 'flexed' (the initial size will not be changed).
         */

        /**
         * @private
         * Calculates the size and positioning of each item in the HBox. This iterates over all of the rendered,
         * visible items and returns a height, width, top and left for each, as well as a reference to each. Also
         * returns meta data such as maxHeight which are useful when resizing layout wrappers such as this.innerCt.
         * @param {Array} visibleItems The array of all rendered, visible items to be calculated for
         * @param {Object} targetSize Object containing target size and height
         * @return {Object} Object containing box measurements for each child, plus meta data
         */
        calculateChildBoxes: function(visibleItems, targetSize) {
            var visibleCount = visibleItems.length,

                padding      = this.padding,
                topOffset    = padding.top,
                leftOffset   = padding.left,
                paddingVert  = topOffset  + padding.bottom,
                paddingHoriz = leftOffset + padding.right,

                width        = targetSize.width - this.scrollOffset,
                height       = targetSize.height,
                availHeight  = Math.max(0, height - paddingVert),

                isStart      = this.pack == 'start',
                isCenter     = this.pack == 'center',
                isEnd        = this.pack == 'end',

                nonFlexWidth = 0,
                maxHeight    = 0,
                totalFlex    = 0,
                desiredWidth = 0,
                minimumWidth = 0,

                //used to cache the calculated size and position values for each child item
                boxes        = [],

                //used in the for loops below, just declared here for brevity
                child, childWidth, childHeight, childSize, childMargins, canLayout, i, calcs, flexedWidth,
                horizMargins, vertMargins, stretchHeight;

            //gather the total flex of all flexed items and the width taken up by fixed width items
            for (i = 0; i < visibleCount; i++) {
                child       = visibleItems[i];
                childHeight = child.height;
                childWidth  = child.width;
                canLayout   = !child.hasLayout && typeof child.doLayout == 'function';

                // Static width (numeric) requires no calcs
                if (typeof childWidth != 'number') {

                    // flex and not 'auto' width
                    if (child.flex && !childWidth) {
                        totalFlex += child.flex;

                    // Not flexed or 'auto' width or undefined width
                    } else {
                        //Render and layout sub-containers without a flex or width defined, as otherwise we
                        //don't know how wide the sub-container should be and cannot calculate flexed widths
                        if (!childWidth && canLayout) {
                            child.doLayout();
                        }

                        childSize   = child.getSize();
                        childWidth  = childSize.width;
                        childHeight = childSize.height;
                    }
                }

                childMargins = child.margins;
                horizMargins = childMargins.left + childMargins.right;

                nonFlexWidth += horizMargins + (childWidth || 0);
                desiredWidth += horizMargins + (child.flex ? child.minWidth || 0 : childWidth);
                minimumWidth += horizMargins + (child.minWidth || childWidth || 0);

                // Max height for align - force layout of non-laid out subcontainers without a numeric height
                if (typeof childHeight != 'number') {
                    if (canLayout) {
                        child.doLayout();
                    }
                    childHeight = child.getHeight();
                }

                maxHeight = Math.max(maxHeight, childHeight + childMargins.top + childMargins.bottom);

                //cache the size of each child component. Don't set height or width to 0, keep undefined instead
                boxes.push({
                    component: child,
                    height   : childHeight || undefined,
                    width    : childWidth  || undefined
                });
            }

            var shortfall = desiredWidth - width,
                tooNarrow = minimumWidth > width;

            //the width available to the flexed items
            var availableWidth = Math.max(0, width - nonFlexWidth - paddingHoriz);

            if (tooNarrow) {
                for (i = 0; i < visibleCount; i++) {
                    boxes[i].width = visibleItems[i].minWidth || visibleItems[i].width || boxes[i].width;
                }
            } else {
                //all flexed items should be sized to their minimum width, other items should be shrunk down until
                //the shortfall has been accounted for
                if (shortfall > 0) {
                    var minWidths = [];

                    /**
                     * When we have a shortfall but are not tooNarrow, we need to shrink the width of each non-flexed item.
                     * Flexed items are immediately reduced to their minWidth and anything already at minWidth is ignored.
                     * The remaining items are collected into the minWidths array, which is later used to distribute the shortfall.
                     */
                    for (var index = 0, length = visibleCount; index < length; index++) {
                        var item     = visibleItems[index],
                            minWidth = item.minWidth || 0;

                        //shrink each non-flex tab by an equal amount to make them all fit. Flexed items are all
                        //shrunk to their minWidth because they're flexible and should be the first to lose width
                        if (item.flex) {
                            boxes[index].width = minWidth;
                        } else {
                            minWidths.push({
                                minWidth : minWidth,
                                available: boxes[index].width - minWidth,
                                index    : index
                            });
                        }
                    }

                    //sort by descending amount of width remaining before minWidth is reached
                    minWidths.sort(function(a, b) {
                        return a.available > b.available ? 1 : -1;
                    });

                    /*
                     * Distribute the shortfall (difference between total desired with of all items and actual width available)
                     * between the non-flexed items. We try to distribute the shortfall evenly, but apply it to items with the
                     * smallest difference between their width and minWidth first, so that if reducing the width by the average
                     * amount would make that item less than its minWidth, we carry the remainder over to the next item.
                     */
                    for (var i = 0, length = minWidths.length; i < length; i++) {
                        var itemIndex = minWidths[i].index;

                        if (itemIndex == undefined) {
                            continue;
                        }

                        var item      = visibleItems[itemIndex],
                            box       = boxes[itemIndex],
                            oldWidth  = box.width,
                            minWidth  = item.minWidth,
                            newWidth  = Math.max(minWidth, oldWidth - Math.ceil(shortfall / (length - i))),
                            reduction = oldWidth - newWidth;

                        boxes[itemIndex].width = newWidth;
                        shortfall -= reduction;
                    }
                } else {
                    //temporary variables used in the flex width calculations below
                    var remainingWidth = availableWidth,
                        remainingFlex  = totalFlex;

                    //calculate the widths of each flexed item
                    for (i = 0; i < visibleCount; i++) {
                        child = visibleItems[i];
                        calcs = boxes[i];

                        childMargins = child.margins;
                        vertMargins  = childMargins.top + childMargins.bottom;

                        if (isStart && child.flex && !child.width) {
                            flexedWidth     = Math.ceil((child.flex / remainingFlex) * remainingWidth);
                            remainingWidth -= flexedWidth;
                            remainingFlex  -= child.flex;

                            calcs.width = flexedWidth;
                            calcs.dirtySize = true;
                        }
                    }
                }
            }

            if (isCenter) {
                leftOffset += availableWidth / 2;
            } else if (isEnd) {
                leftOffset += availableWidth;
            }

            //finally, calculate the left and top position of each item
            for (i = 0; i < visibleCount; i++) {
                child = visibleItems[i];
                calcs = boxes[i];

                childMargins = child.margins;
                leftOffset  += childMargins.left;
                vertMargins  = childMargins.top + childMargins.bottom;

                calcs.left = leftOffset;
                calcs.top  = topOffset + childMargins.top;

                switch (this.align) {
                    case 'stretch':
                        stretchHeight = availHeight - vertMargins;
                        calcs.height  = stretchHeight.constrain(child.minHeight || 0, child.maxHeight || 1000000);
                        calcs.dirtySize = true;
                        break;
                    case 'stretchmax':
                        stretchHeight = maxHeight - vertMargins;
                        calcs.height  = stretchHeight.constrain(child.minHeight || 0, child.maxHeight || 1000000);
                        calcs.dirtySize = true;
                        break;
                    case 'middle':
                        var diff = availHeight - calcs.height - vertMargins;
                        if (diff > 0) {
                            calcs.top = topOffset + vertMargins + (diff / 2);
                        }
                }

                leftOffset += calcs.width + childMargins.right;
            }

            return {
                boxes: boxes,
                meta : {
                    maxHeight   : maxHeight,
                    nonFlexWidth: nonFlexWidth,
                    desiredWidth: desiredWidth,
                    minimumWidth: minimumWidth,
                    shortfall   : desiredWidth - width,
                    tooNarrow   : tooNarrow
                }
            };
        }
    });

    Ext.Container.LAYOUTS.hbox = Ext.layout.HBoxLayout;