@quik I'm happy this is a work around, however this should be a setting option somewhere in Opera itself. All other browsers work this way by default, and I honestly uninstalled Opera just because this was annoying me.
Also in your recording, you struggle for a second to get the tabs to join. Upping the threshold on this would be fantastic. If there's no tabs left, its nearly impossible to drag a tab over to that window so why not close it by default? What ends up happening is you get multiple open opera instances with no tabs.
I want to love your client, and you're 99% there. Please reconsider your stance on this issue. It would mean a lot to those who want to switch or tried to. The most important thing for those coming from another environment is that it feel familiar and even if they're unsure exactly why something feels off, things like this will push people away. I get its small, and I get wanting to be unique to set yourself apart, but I would be willing to bet this turns more people away than you realize.
Using the above workaround I did reinstall and I'm enjoying your browser much more. Please consider adding this as a core feature change.