grammaticality - When is "Your" grammatically correct instead of "You are"

So a while ago, I edited an answer on meta, wondering Your should be replaced with You are. Here is an excerpt from the post:

You are [Your] not being able to cast another close vote is also officially by design. We only let users vote to close once, whether the question is successfully closed or not. In this particular case, it might be useful for the vote to be cleared in such a way as if it had never existed at all, thus allowing you to vote to close again for a different reason........

It was later revised stating, "Your" is grammatically correct there. I believe he might be right as he is a moderator. Though I would want to know the reason behind it.

I've been googling for a while now and followed these posts:

But still don't know how come this is grammatically correct?