I'm having a problem similar to @lancealot on Linux KDE. Random tabs pop to the top of the window stack and take focus. Not sure what triggers it since the raised tabs seem completely loaded, but it's a really bad UX. This started happening in maybe the last couple weeks. The problem is reduced by fiddling with "focus stealing prevention" settings, but that has not been necessary before with Opera and I suspect it should not be necessary now.
Hope the problem can be fixed. Thanks for an otherwise pretty great browser!