Comment (by Roland Haas):

On closer inspecting this may be correct, but benefit from a comment. Namely, not having a return statement in 678 means that (due to the assert in 677) the code is guaranteed to clear sf_valid in 683 and return there.

Ie the code will mark the surface that was not looked for as invalid.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2337/ahfinderdirect-does-not-mark-surfaces-as