Hi Basj,
Here's the video:
https://drive.google.com/open?id=1ImDjPnO-vzngjUHJOdJjVmuVYRFvRP42
You can see that when I drag tabs from one window to another it usually works, but sometimes it snaps to the other window. The video also shows the following behavior:
1. After moving a Group or Tab from one window to another, changing tabs can snap the Group back to the other monitor.
2. Attempting to snap a Group to the top of one of the screens resulted in it being snapped to the other screen.
This suggests that Groupy isn't properly tracking which monitor each program should be on. Instead of updating those values when switching monitors, sometimes it uses the value of where the program used to be.