Not sure if this is because I'm on an old version or not, but if I open the game and then change tabs battles continue to go on, but I never advance levels in the battle screen. I noticed when I woke up with 1400 scrap. 😅
I played around with this a bit. Levels seemed to increase for me even while in another tab. That said, there's always some quantum weirdness about out-of-focus browsers. Did you also notice abnormally high kill counts (more than 330) in the battleground when you came back? It's not unreasonable that you could have gotten 1400 scrap just from having a bunch of items in the queue to be collected and scrapped.