The source code

    /**
     * @class Ext.layout.VBoxLayout
     * @extends Ext.layout.BoxLayout
     * <p>A layout that arranges items vertically down a Container. This layout optionally divides available vertical
     * space between child items containing a numeric <code>flex</code> configuration.</p>
     * This layout may also be used to set the widths of child items by configuring it with the {@link #align} option.
     */
    Ext.layout.VBoxLayout = 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:
         * <div class="mdetail-params"><ul>
         * <li><b><tt>left</tt></b> : <b>Default</b><div class="sub-desc">child items are aligned horizontally
         * at the <b>left</b> side of the container</div></li>
         * <li><b><tt>center</tt></b> : <div class="sub-desc">child items are aligned horizontally at the
         * <b>mid-width</b> of the container</div></li>
         * <li><b><tt>stretch</tt></b> : <div class="sub-desc">child items are stretched horizontally to fill
         * the width of the container</div></li>
         * <li><b><tt>stretchmax</tt></b> : <div class="sub-desc">child items are stretched horizontally to
         * the size of the largest item.</div></li>
         * </ul></div>
         */
        align : 'left', // left, center, stretch, strechmax
        type: 'vbox',

        /**
         * @cfg {String} pack
         * Controls how the child items of the container are packed together. Acceptable configuration values
         * for this property are:
         * <div class="mdetail-params"><ul>
         * <li><b><tt>start</tt></b> : <b>Default</b><div class="sub-desc">child items are packed together at
         * <b>top</b> side of container</div></li>
         * <li><b><tt>center</tt></b> : <div class="sub-desc">child items are packed together at
         * <b>mid-height</b> of container</div></li>
         * <li><b><tt>end</tt></b> : <div class="sub-desc">child items are packed together at <b>bottom</b>
         * side of container</div></li>
         * </ul></div>
         */

        /**
         * @cfg {Number} flex
         * This configuation option is to be applied to <b>child <tt>items</tt></b> of the container managed
         * by this layout. Each child item with a <tt>flex</tt> property will be flexed <b>vertically</b>
         * according to each item's <b>relative</b> <tt>flex</tt> value compared to the sum of all items with
         * a <tt>flex</tt> value specified.  Any child items that have either a <tt>flex = 0</tt> or
         * <tt>flex = undefined</tt> will not be 'flexed' (the initial size will not be changed).
         */

        /**
         * @private
         * Calculates the size and positioning of each item in the VBox. 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,
                availWidth   = Math.max(0, width - paddingHoriz),

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

                nonFlexHeight= 0,
                maxWidth     = 0,
                totalFlex    = 0,
                desiredHeight= 0,
                minimumHeight= 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, flexedHeight,
                horizMargins, vertMargins, stretchWidth, length;

            //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 height (numeric) requires no calcs
                if (typeof childHeight != 'number') {

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

                    // Not flexed or 'auto' height or undefined height
                    } 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 (!childHeight && canLayout) {
                            child.doLayout();
                        }

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

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

                nonFlexHeight += vertMargins + (childHeight || 0);
                desiredHeight += vertMargins + (child.flex ? child.minHeight || 0 : childHeight);
                minimumHeight += vertMargins + (child.minHeight || childHeight || 0);

                // Max width for align - force layout of non-layed out subcontainers without a numeric width
                if (typeof childWidth != 'number') {
                    if (canLayout) {
                        child.doLayout();
                    }
                    childWidth = child.getWidth();
                }

                maxWidth = Math.max(maxWidth, childWidth + childMargins.left + childMargins.right);

                //cache the size of each child component
                boxes.push({
                    component: child,
                    height   : childHeight || undefined,
                    width    : childWidth || undefined
                });
            }

            var shortfall = desiredHeight - height,
                tooNarrow = minimumHeight > height;

            //the height available to the flexed items
            var availableHeight = Math.max(0, (height - nonFlexHeight - paddingVert));

            if (tooNarrow) {
                for (i = 0, length = visibleCount; i < length; i++) {
                    boxes[i].height = visibleItems[i].minHeight || visibleItems[i].height || boxes[i].height;
                }
            } 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 minHeights = [];

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

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

                    //sort by descending minHeight value
                    minHeights.sort(function(a, b) {
                        return a.available > b.available ? 1 : -1;
                    });

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

                        if (itemIndex == undefined) {
                            continue;
                        }

                        var item      = visibleItems[itemIndex],
                            box       = boxes[itemIndex],
                            oldHeight  = box.height,
                            minHeight  = item.minHeight,
                            newHeight  = Math.max(minHeight, oldHeight - Math.ceil(shortfall / (length - i))),
                            reduction = oldHeight - newHeight;

                        boxes[itemIndex].height = newHeight;
                        shortfall -= reduction;
                    }
                } else {
                    //temporary variables used in the flex height calculations below
                    var remainingHeight = availableHeight,
                        remainingFlex   = totalFlex;

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

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

                        if (isStart && child.flex && !child.height) {
                            flexedHeight     = Math.ceil((child.flex / remainingFlex) * remainingHeight);
                            remainingHeight -= flexedHeight;
                            remainingFlex   -= child.flex;

                            calcs.height = flexedHeight;
                            calcs.dirtySize = true;
                        }
                    }
                }
            }

            if (isCenter) {
                topOffset += availableHeight / 2;
            } else if (isEnd) {
                topOffset += availableHeight;
            }

            //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;
                topOffset   += childMargins.top;
                horizMargins = childMargins.left + childMargins.right;

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

                switch (this.align) {
                    case 'stretch':
                        stretchWidth = availWidth - horizMargins;
                        calcs.width  = stretchWidth.constrain(child.minWidth || 0, child.maxWidth || 1000000);
                        calcs.dirtySize = true;
                        break;
                    case 'stretchmax':
                        stretchWidth = maxWidth - horizMargins;
                        calcs.width  = stretchWidth.constrain(child.minWidth || 0, child.maxWidth || 1000000);
                        calcs.dirtySize = true;
                        break;
                    case 'center':
                        var diff = availWidth - calcs.width - horizMargins;
                        if (diff > 0) {
                            calcs.left = leftOffset + horizMargins + (diff / 2);
                        }
                }

                topOffset += calcs.height + childMargins.bottom;
            }

            return {
                boxes: boxes,
                meta : {
                    maxWidth     : maxWidth,
                    nonFlexHeight: nonFlexHeight,
                    desiredHeight: desiredHeight,
                    minimumHeight: minimumHeight,
                    shortfall    : desiredHeight - height,
                    tooNarrow    : tooNarrow
                }
            };
        }
    });

    Ext.Container.LAYOUTS.vbox = Ext.layout.VBoxLayout;