A fullscreen header is a header with a position of "fixed" except that the header overlays the page content, rather than reserving a place in the document. This is useful for immersive apps like photo or video viewers where you want the content to fill the whole screen and toolbars can be hidden or summoned to appear by tapping the screen. Keep in mind that the header in this mode will sit over page content so this is best used for specific situations.
This property is ignored unless headerPosition is set to "fixed".
True, False, or any valid XPath expression that returns a boolean value.