There has been a lot of focus on grouping of tabs in browsers lately as a new and wonderful way to manage tabs, but did you know that you have actually been able to group tabs in Opera for many years already? …
It might not be obvious that it can be used for this purpose, but the "Windows" panel in Opera shows all your open windows, and the tabs within each window. It's disabled by default, so open the panel selector and click the "+" at the bottom to find the panel.
You may notice that you can collapse and expand groups/windows, and the group is named after the currently active tab in that group. You can also drag and drop tabs between groups/windows, and rearrange them within each window. Drag a tab outside a group/window to create a new one. The "Quick find" field lets you filter tabs in real-time.
Addition: Combine tab grouping with real-time filtering for added productivity!
Opera can remember all your open windows on exit, but to do so, you should use "Exit" from the menu, rather than pressing Alt+F4 or the X that closes the program window. Luckily, Opera can reopen closed windows, so you will always have a way to get them back.
So there you have it. Basic tab grouping in Opera that has existed for years. I'm sure there are many possible improvements that could be made to Opera's tab grouping, but we'll leave that for the wish-list forum 😉