tbh though, I've had so many bugs with tabs-windows I'm done using them. I'm just going to mass-bookmark my tabs and delete all my saved windows.
edit: just lost half my saved tabs after trying to move them to the undeletable, unnamed "main" window, getting this behavior again (without editing either window), and restarting the app. My last remaining named window reopened with an assortment of tabs from its previous incarnation - some that I had "moved" to the unnamed window, some that I had not yet moved - and the rest are gone forever.