This approach is more flexible in that it allows even the local participant to share their screen in CallViewModel tests, and more rigorous in that it ensures that application code is reacting specifically to track publications.
While looking into what had regressed https://github.com/element-hq/element-call/issues/3588, I found that 28047217b8 had filled in a couple of behaviors with non-reactive default values, the "natural window mode" behavior being among them. This meant that the app would no longer determine the correct window mode upon joining a call, instead always guessing "normal" as the value. This change restores its reactivity.
