how to make facebook theater view

[Perica]

when you click photo on facebook it show pop up, it's look like can be done with lightbox or something like that, look at address bar, the url was changed, it can be done with pushState javascript methodBUT do you look at the browser vertical scrollbar? it's like opening new page when the page doesn't overflowI'm sure its not using \[code\]body{ overflow:hidden} \[/code\]because when you open Developer Tools (in Chrome) that scrollbar is activeIt just like a whole new page, I hope you understand my question, sorry for bad english