Good question :)
Screenbox is not really a separate layout, but an "experimental" add-on style we created for the standard
vertical layout. The idea is that each image canvas is scaled to take exactly 100% of the visitors viewport (visible screen area), and images are scaled to fit inside (90% height or width, whichever is limited first). We never really extended the concept, because it did not work properly in several mobile browsers, and it has some limitations (you cannot include captions, unless they are overlaying images). However, that was some time ago, and maybe it's time to bring this concept public again.
To enable it, you can do the following from page gallery settings:
- Choose "vertical layout".
- Uncheck "horizontal rule".
- Set "wide" gallery width.
- Under styles and effects, type in "screenbox-90".