Flex borders

The idea of the flex border is that it doesn't dictate the size of the contained gadget. If you construct the normal <border>, the contained gadget is no longer resized with the parent, because the <border> tries to keep it in the middle, which also doesn't propagate the resize requests to the child. The <flex-border> is composed of <track>'s, which give full size to the child gadget, but also (optionally) add fixed-size empty gadgets to pad the child from top and bottom and/or from the sides.
<flex-border> ( gadget gaps -- gadget' )