You say the pax placed his bag in front of you, but the FA said it was between you two. Which one was it?

I generally consider it even split, half of the underseat space mine, half of it my seatmate's. If his bag was really in your underseat space such that it would restrict you stretching out your legs, then I find that ridiculous, even more so that a first class pax would do such a thing.

I would've replied with "well, I want my legroom too" and then stretch out my legs as if his bag wasn't there. Or maybe use his bag as a footrest. We'll see if he moves it then.