I thought it one of the really interesting parts of the Sets experiments that the Windows team was working on (and ultimately seem to have cancelled). They fell straight into this rabbit hole of "sessions" and defining what they mean to a user as soon as they tried adding (heterogeneous) Tabs as a native UI element to every window title bar. A lot of the dev talk on what to expect with Sets was on trying to surface user-meaningful activities to the Microsoft Graph and thus to the Windows Timeline thus making those groups of tabs meaningfully relevant to a user ("these are the tabs I used to file my expense report").

It is definitely a hard problem to solve, especially if you are hoping for software to do a lot of that aggregation automagically "reading people's minds". It was really cool seeing some very smart people on the Windows team trying to build it with Sets, and maybe all the more disappointing that the idea didn't make it "out of the lab" for its grand attempts at trying to build a more complicated/nuanced/powerful model of how people interact with groups applications at a time.