I went to the user's profile and right-clicked their photo. The contextmenu action is intercepted and replaced with a custom menu mimicking the browser's context menu. This fake context menu has the option "Open in Window". I click it. The website doubles-down on the charade and opens a "popup" inside itself - maximize/minimize icons in the action bar and everything.
It's the sort of thing you're taught phishing websites do to lure you into inputting your credentials into the wrong website.