Window overlapping fullscreen window

I recently had an incredibly useful experience where I was able to take a window from my desktop and place it over top of one of my fullscreen windows. Since then, I haven't been able to replicate it or find anything that even says this feature exists.

Both windows were Chrome. It wasn't side by side view.

When I did whatever I did, it looked like I was in mission control (where you swipe up with three or four fingers), except that the window I had been on was present as a rectangle above my desktop. I moved the window over top of one of my fullscreen windows, and the fullscreen window let me drop it in (not as a side by side).

After that, the window I had dropped in functioned like a normal window on the desktop, but it always stayed on top of the fullscreen window. I accidentally ended it when I clicked fullscreen on the regular window and it changed into its own fullscreen on another window.

Was this a bug? Can it be replicated? (It was really useful.)