Special ViewPort region

There is a special reference region named ViewPort that is not a region in the layout but can be used as a reference region for placement specifications.

This ViewPort region is the name of the browser window. You can consider the four edges to be the inner edges of the browser panel that contains the page content. When the user scrolls the page, regions placed relative to the ViewPort behave in a special way. They stay fixed to the edges (or middle) of the ViewPort while the content underneath them moves.

If the layout is inside of an iframe, the ViewPort is the edge of the iframe.