How to keep one chrome window open infront of another

I was working on something, and accidentally I managed to get it so I had two windows open infront of another, and they would both stay open in that position regardless of which one I was working on. I have never managed this before, I didn't know it was possible.

I tabbed out and in again and lost the magic. How do I do this again?

Adding photos to show what I mean:

I want the top, smaller window to stay open in that position even when I click on the bigger window behind it, without the front collapsing.