Basically, it closed a buffer because the backing file disappeared when I checked out a different branch. So I guess the lesson is use your file system to retain data and never your editor, which sounds obvious and you could certainly argue the loss was my fault. But I do that all the time with Sublime and never had this problem once. In fact I could quit Sublime altogether and the data would still be there when Sublime reopened.

Basically, I can see how the choice they made is a reasonable one, but my stance is a buffer should never close until I say so. If the backing file disappears, the last contents of the buffer should stay in there until I close it.

I bet a lot of people will never get bit by this, but it's nasty enough that I think they should reconsider this decision. But for me it doesn't matter, I don't want to lose work because I trusted my editor to hold on to something and it didn't, so I'll likely never use it again.