Well, I find the idea certainly acceptable, since I've found myself asking then and again "can you type exactly (!VER), w/e". But I find pop-up dialogs very annoying.
What do you think about a message with two links, as if it was an invitation to something, but it's just a Plus rendered text. As usual, two links with 'Accept' and 'Refuse'. It would also match receiver's Plus! language, as the messages would be pre-configured.
But, I'm sorry to say this feature delies to much on the fact that both sender and receiver must have Plus! installed to make it work. Patchou isn't yet willing to add Plus!2Plus! features to MP! since the addon itself doesn't track (automatically) who and who doesn't have Messenger Plus!
So I'm afraid that wouldn't be added.
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