align:'stretch' config of the vbox layout manager ensures
all child items are 100% of the container width.
flex config of child items of a vbox layout specifies what share of the vertical
space left when unflexed items are accounted for to allocate for that child item.
Compare this to The anchoring example. That anchors the fields to a hardcoded offset from the edge of the Container. The box layout managers allow much more flexibility in use of available space.
The js is not minified so it is readable. See vbox-form.js.
This also illustrates the use of plugins to alter the default behaviour of Components. The Send To field clones itself until the final one is left blank to allow multiple mail recipients. The layout manager keeps the vertical space allocated correctly.