I really can't believe this feature still hasn't been integrated in Opera. Like others have noted, I am really struggling to continue working in this browser without this fundamental function.
I did want to add a solution that has been working for me. I tested out several Chrome extensions, and this one ("Move tabs between windows", linked below) is the only one I have found so far that actually works in Opera for moving multiple tabs at a time between open windows. I was specifically looking for an extension that moved tabs to other open windows, rather than just moving them all to a new window. To use this extension, select all the tab you need moved (holding down the CTRL/CMD as you select tabs for multi-select). Then press the extension in the toolbar, and open the window you would like the tabs moved to. After changing focus to the desired destination window, the tabs should appear in that window.
https://chrome.google.com/webstore/detail/move-tabs-between-windows/dcgkknngicjdagcjljfbhgdimlfhafag/related?utm_source=chrome-ntp-icon
Unfortunately, this extension still doesn't replace the option to drag and drop tabs like normal, especially if you are simply trying to rearrange tabs within the active window (like some users note on here: https://forums.opera.com/topic/34453/suggestion-able-to-select-multiple-tabs-at-once-and-move-the-all-together/9). I really like using Opera and would hate to leave just because of this, so please add this feature!
