Programming on the Oculus Rift.
They were working on developing games and experiences for virtual reality, by way of Facebook's own Oculus Rift VR headset.
But having to put the Rift goggles on every ten seconds to see the effects of the changes they made to the code, and then taking them off again to get back to work on a computer monitor, was slowing down work.
The solution, as put together by two engineers during one of Facebook's famous hackathons, was simple: a system that renders the programming interface ("integrated development environment," or "IDE," in programmer-speak) in virtual reality.
Built using Facebook's own React app framework, which is appreciated by programmers within and without the social network for its flexibility across platforms, this hack made VR development a little more science fictional.
Coders can look around and create simple objects, like boxes or spheres, that appear in virtual reality.
Beyond mere "Minecraft"-like building status, though, the presence of that floating programming window - used via a normal keyboard in meatspace - means that Facebook engineers get full access to the all-important code underneath. Change the code, and it updates in real-time, all rendered in glorious three-dee virtual reality.
It looks a little primitive. But it elicits images of programmers building their own little worlds in virtual reality, in a scene that's like William Gibson's "Neuromancer."
Facebook just explained the hack in blog post outlining the company's favorite hackathon-born features of the year. Other honorable mentions went to the Instagram Boomerang app and the mega-popular "Pride Filter" to celebrate the landmark gay marriage ruling earlier in 2015.