Class handling popup layout defined by setting layout.type to "popup". The popup layout model
does not render it's children inside the "body" of it's parent. Instead, it's rendered as direct
children of document.body(<body>). Layout properties are used to measure size and
position. One example of use is a combo box which displays a child view below a button or input box.
See {{#crossLink "layout.LayoutSpec"}}{{/crossLink}} for the available position and
sizing properties available to children inside a popup layout.