The definitions of "unit" has changed over time. The idea (and I mean the IEEE definition of the idea that has since changed) used to be more based in OOP and the idea was that "units" were larger pieces of an app that interacted with other larger pieces of an app. The idea being that it was at those points that the highest risks were incurred by changes so those spots were most worth keeping an eye on as apps changed over time. It's almost as if they thought it might be a waste of time to let the testing of every corner of an app drive their architecture away from legibility, ease-of-modification, and reuse, to the altar of everything being easily tested.
Since then, reason has prevailed and it has been determined that we should test every smallest piece of our apps (the modern "unit") possible to make sure nobody has made a misstep.
But what were those devs from the '80sish-90sish thinking?
Perhaps they were under some delusion that you couldn't possibly check every single assumption with another assumption, but that you could at least check spots where Object A was supposed to shake hands with Object B with a variety of different hands/greetings/circumstances/etc and see if there was an unexpected miss due to some change made that didn't account for some interaction required by some other aspect of the app that the change-making dev was less familiar with. Mad they were, those people who originally coined the term "unit testing" because clearly, we must not only test our own code but also the code in frameworks that are used by tens of thousands daily and even the browser APIs which are tested by billions daily by accident.
IMO, you should map every single possible entry/exit point of whatever framework you are using so that you can mock every last piece of it and tell whoever is pressuring you to get 100% code coverage that you are indeed doing so and even though all of your estimates just went up by 10 orders of magnitude, that holy !@#$ are you guys going to be covered in case there is the slightest misstep made by browser developers, framework developers, or anybody on your team and they can rest assured that no mistakes will have been made on their watch, by the developers.