After understanding this is not a race condition between the ViewPager, and the drawer, I've dug into the page, t find out that Facebook customize the page width according to some parameters which I cared not about...
Later I've installed user agent switcher extension to Chrome, and been able to reproduce it, with a my custom user agent.
I did not expect the user agent to have such an effect on a page...
But hell, I've been wrong a billion times before!