You found the problem Phillip! My editor was trimming trailing white space, which breaks the context line. I had tried to use an alternative editor to account for any editor specific behaviour, but it turns out both the editors I tested in were doing this! I suspect this change in behaviour will effect a lot of users? If so, it would be good if `git add -p` allowed for this behaviour, in the same way `git apply` does. Meanwhile, I can easily configure my editor not to do this for `*.diff` files. Thanks for your help, Phillip and Martin! Mahmoud, does this also explain your problem as per your original post?
