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Imho the samsung dex approach is a fine concept (not saying that was fully developed or as extensive as what would be required to do it seriously for a proper desktop os)

Two totally seperate UIs that can run at the same time, a desktop one and a phone one.. it worked shockingly well imho.

Maybe an additional case where you can use the tablet as an extra screen, but honestly that could probably manifest as an app in the mobile UI.