This behavior can definitely be frustrating, especially for users who regularly organize tabs across multiple windows. Hopefully Opera provides an option to choose between attaching tabs to the current window and opening them in a new window, giving users more control over tab management. There have been similar discussions on the Opera forums as well, such as: https://forums.opera.com/topic/62175/check detailsduplicated-impossible-to-drop-a-tab-in-another-opera-window.