One can use the copula to connect noun phrases of different number.
The conversational topic that kept us pleasantly chatting was the different Southern dialects in the US.
Here it sounds far clear that "is" should be singular. When the order of the NPs is reversed, it's a little less clear that "is" needs to become plural:
The different Southern dialects in the US was the conversational topic that kept us pleasantly chatting.
My question is, Is this simply an artifact of the order in which I came to this example, or is the ability to bundle multiplicities into singular entities behind this? And, do stylistics handbooks opine on the issue?
PS: I tried to do some searching around for information about this, but without knowing how to talk about copulation (e.g. are the actors of copulation called "copulands"?) it was difficult to get anything useful.