I installed the pop up pet door, made some adjustments. Cut the bottom bit off the flap and then slit it up the middle. And put it on the door. Then I closed the slider to see how the gap was and while I was checking that...
Biggie marched right through the flaps to the terrace.
So, that worry was a waste.
I found some foam with tape backing and so was even able to fill the gap nicely where the slider and cat door don't quite match.
It's not perfect but it's sure good enough. If we had horribly cold winters, or if I had issues heating the house, it wouldn't work well enough. But we don't and I don't and it's going to be fine.
I am going to be in a bit of a pickle next June when I need to set up the air conditioner and its vent but, I'm going to Scarlett O'Hara that sitch. I'll think about that tomorrow. Tomorrow is another day.