How to protect against adversaries snatching booted laptops to defeat full disk encryption?

I read an article describing how FBI agents snatched Ross Ulbricht's laptop while it was running to defeat full-disk encryption:

Two plainclothes FBI agents, one male and one female, walked up behind Ulbricht and began arguing loudly. This staged lovers' tiff caught Ulbricht's attention long enough to distract him from his laptop. As soon as Ulbricht looked up, the male agent reached down and slid the computer over to his female colleague, who quickly snatched it up and handed it over to Kiernan for further investigation.

Ideally, one would only operate computers from places that are more physically secure than a public library, but that is not always practical in the real world.

I imagine that journalists working in dangerous places and other well-meaning people are also vulnerable to the theft of running laptops (thus defeating full-disk encryption), and I would like to figure out how to help them.

What kind of security measures can protect the data on a running laptop being physically snatched?