Opera has so many things built-in that it is tough to find and use all it has to offer. After you find out about the features they become incredibly useful, and this is a great example of that.
Panels in Opera are essentially sidebar items, such as the bookmark sidebar. By default Opera comes with 12 Panels that all perform various tasks, such as displaying file transfers and managing bookmarks. One Panel that I didn’t know about is called “Windows,” and I have ResearchWizard to thank for pointing it out!
The Windows Panel does not come enabled out-of-the-box, but it can be added by right-clicking anywhere on the Panel bar and choose the Customize option. Once you have it enabled you’ll be able to do a lot of awesome stuff with your all of your Opera tabs, even ones that are open in other windows:
All of your tabs will be arranged according to which window they currently belong to, but you can drag and drop them anywhere you want. You can even select multiple tabs using the Ctrl and Shift keys to move them in bulk! I really like this because I can keep multiple windows open and drag tabs over as I need them, and if I can’t remember what a tab was about I can just hover over the title to see a live preview of the site.