From Carlos in bug #762812:

"""We need to use the same page, but with elements hidden, instead of using a different page. So that when something is loaded we just hide the initial page and show the overview. To see what I mean do the following:

 1.- Open a new ephy
 2.- Do not load anything in the initial page, open a new tab
 3.- Load something in the new tab
 4.- Visit the overview (initial) tab

It's kind of mixed. The web extension should remove the overview-empty class, or use display none, or whatever, it depends on how we do the html thing. I think it's easier to use the same html code and simply add remove classes to make it work as empty non empty."""

Not that "open a new epyh" means passing --profile with some directory such that you get an instance with no history.