came up with a new idea

context: I and you attend a lecture. You think of an idea. Three days later, I say:

It is/was you who came up with the new idea!

Which one should be used?

IS seems OK, because it IS NOW you who came up with the new idea IN THE PAST

WAS seems OK, because I am talking about a past event.

It was you.

Thank you.

So you think IT IS YOU illogical?

Since the "coming up" with the new idea is in the past, "was" seems right to me.

If it was a continuing thing, then "is" might work as well.

"It's you that believes in space aliens, right?"

Thank you. I've learnt a lot.

How about this similar situation?

context: A teacher tells students to have a discussion. One minute later. The discussion ends. The teacher asks one of the students to give a presentation on what they have just said.

What should I use?:

What we were talking about is/was that....

Or is there any mistake(eg, WERE→HAVE BEEN)in the sentence, or is there a better or more natural way to express the same meaning?

I don't really think that students are going to worry too much about the appropriate tense to use one minute after a discussion has ended

Thank you.

But somehow I am very often chosen to say such a thing by the professor of a lecture.

You think in What we were talking about is/was that....

both are acceptable?

Were talking ... was
Are talking ... is