Tabs are not easy to handle while you separating or merging them to/from Opera windows. Sometimes you have to wait too much time before tab are merged to windows or you have to really precise pointing them to really specific location (only to the left side of the "+" button) on the tab bar before they finaly merge. Also when you separate last tab from the window the window not dissapear but create new home screen instead. Also separating tab from window could be much better and when you grab it it could be chaged to separate window right away not just after you drop it by mouse.