fix: avoid viewport height for fullscreen Reveal as it may be incorrect #10879

fix: avoid viewport height for fullscreen Reveal as it may be incorrect #10879

The CSS viewport is the larger possible view height and does not change with the visible area height. This is an intended behavior for optimization reason. See this article:

See: https://nicolas-hoizey.com/2015/02/viewport-height-is-taller-than-the-visible-part-of-the-document-in-some-mobile-browsers.html
See: https://github.com/zurb/foundation-sites/issues/10879#issuecomment-414087274

Closes https://github.com/zurb/foundation-sites/issues/10879