that's a pretty genius use of the "portals" primitive; I suppose its akin to Portals in React-ish.

Although I've never needed to open 2 tabs of the same site and wanted them share the state.

For me the main use case is screen sharing (share only the mirrored window). It can also be useful for multi-monitor setups when you want to show something to other people on a bigger screen without mirroring your main screen.