Pretty awesome
http://prototype-imdb.webflow.com
Looks like a block set to absolute positioning regarding the Body, with a padding on the body to ofset the site to the right. The drop down is an interaction revealing a sliding pane.
It’s really a showcase, not a proof of concept, if you were to try to make a whole WF site to work like this it would require a lot of work in interactions 
Not impossible at all though.
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